Bump typescript from 3.9.3 to 4.2.4

[0xdevalias]

closes https://github.com/sparkletown/internal-sparkle-issues/issues/160

---

We're currently using "typescript": "^3.9.3":

• https://github.com/sparkletown/sparkle/blob/staging/package.json#L156

The latest version is 4.2.4:

• https://github.com/microsoft/TypeScript/releases/tag/v4.2.4

There have been a number of releases since our current version, each adding new features, and their own 'breaking changes' categories:

• https://devblogs.microsoft.com/typescript/announcing-typescript-3-9/ (this is the version we are currently on)
  • ⚠️ ⚠️ ⚠️ https://devblogs.microsoft.com/typescript/announcing-typescript-3-9/#breaking-changes (so these probably don't matter here)
• https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/
  • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#breaking-changes
    • Nothing much that sounds like it will impact us
  • Notable new features
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#variadic-tuple-types
      • Useful
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#labeled-tuple-elements
      • Also useful for better dev efficiency/documentation/usability
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#class-property-inference
      • Could help us avoid situations where the typing becomes an 'implicit any'
      • If we don't already, we should enable noImplicitAny
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#short-circuiting-assignment-operators
      • While we don't really use mutable state very often as a 'rule', this could have it's uses
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#unknown-on-catch
      • This would be a good feature to use to prevent errors in our error handlers
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#custom-jsx-factories
      • Probably not all that useful for us, but interesting to note
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#build-and-noemitonerror
      • Not sure if this relates to how we use it, but good to know in case it does
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-0/#noemit-and-incremental
      • Not sure if this relates to how we use it, but good to know in case it does
• https://devblogs.microsoft.com/typescript/announcing-typescript-4-1
  • ⚠️ ⚠️ ⚠️ https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#breaking-changes
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#any-unknown-are-propagated-in-falsy-positions
      • This one is worth paying attention to. Not sure if it would impact our code at all.. but calling it out in case.
  • Notable new features
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#template-literal-types
      • This could come in handy
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#key-remapping-in-mapped-types
      • Also could be quite useful
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#recursive-conditional-types
      • And this
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#checked-indexed-accesses-nouncheckedindexedaccess
      • This is going to be SUPER useful, and remove a whole bunch of potential bugs without us needing to be overly verbose with our typing
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#paths-without-baseurl
      • This will be useful to check/enable (if it isn't on by default)
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#react-17-jsx-factories
      • I believe this was probably one of the blockers stopping us from upgrading create-react-app to latest (see Bump react-scripts from 3.4.1 to 4.0.3 + fix associated tech debt #1102)
      • We probably want to look into this a little, and update our config for it (see also Add emit support for jsx/jsxs experimental jsx runtime api microsoft/TypeScript#39199)
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-1/#editor-support-for-the-jsdoc-see-tag
      • Useful for improving our comments/usability of them within IDEs
• https://devblogs.microsoft.com/typescript/announcing-typescript-4-2
  • ⚠️ ⚠️ ⚠️ https://devblogs.microsoft.com/typescript/announcing-typescript-4-2/#breaking-changes
    • Nothing much that sounds like it will impact us
  • Notable new features
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-2/#smarter-type-alias-preservation
      • This will likely make it MUCH easier to understand type errors!
    • https://devblogs.microsoft.com/typescript/announcing-typescript-4-2/#unused-destructured
      • This is also a nice improvement

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