Add (optional) type declaration checks in app, src

[colinrotherham]

This PR adds a tsconfig.json so the TypeScript compiler can check our code and closes:

• Annotate our code with types and lint to ensure their proper usage #3230

What does this mean for us?

1. Checks we're calling library code as intended
2. Checks we're calling DOM methods as intended
3. Checks we're adhering to our own JSDoc comments

GitHub Actions

Checks are optional and can be run manually

For example, missing or invalid parameters will be spotted by the compiler:

  src/govuk/components/character-count/character-count.mjs:59:35
    59 function CharacterCount ($module, config) {
                                         ~~~~~~
    An argument for 'config' was not provided.

Related changes

This PR completes work we started for v4.4.0:

• Enable ESLint JSDoc checks #2913
• Various JSDoc + type checking fixes (for v4.4.0 only) #2997

To assist with code reviews I've split these changes into:

1. Ensure JSDoc blocks are complete #3102
2. Configure ESLint to require JSDoc @params etc #3103
3. Various JSDoc + type checking fixes #2987
4. Enable type declaration guards #3123
5. Add (optional) type declaration checks in app, src #3104

Why TypeScript?

We're not actually writing any TypeScript code but this PR includes it to:

1. Check type information via ESLint using standard standard-with-typescript
2. Run the TypeScript compiler tsc to check if our code would build without errors

Similar to source maps, we may want tsc to output type declarations in future:

• Export type declarations for component source code #2835

[colinrotherham]

@romaricpascal I've moved the "mandatory" bits to:

• Add (mandatory) type declaration checks #3319

[romaricpascal]

All ready to go! Thanks so much for your patience with this one for all the reviewing, amends and rebasing to keep it up to date 😄