feat!: adds support for ESM and Deno

[bcoe]

Using @mleguen and @QmarkC's work as a starting point, this refactor adds support for ESM and Deno.

The approach taken is as follows:

1. TypeScript now targets es2015 module format.
2. rollup is used to create a bundled CJS build of the library, including an index.cjs.d.ts (which is the binding that will be used for require statements).
3. conditional exports are used to target both ESM and CJS.

Todo:

• finish updating tests for tokenize-arg-string.ts.
• document in README how to use deno and ESM.
• investigate publishing to pika (I've added a module field, which I believe should allow us to start showing up on pika).
• add config loading support to Deno.
• make config loading work for ESM module.
• make config loading throw reasonable error in browser.
• get test coverage working for bundle.
• configure release-please to push deno branch on release.

BREAKING CHANGE: decamelize and camelCase logic has been re-implemented, and may have some logical differences. Conditional exports for CJS vs., ESM may break some environments.

[jkrems]

Is this a bug in rollup's support for --exports=default? If so, is there an open issue this could link to track when the workaround may become unnecessary?

[bcoe]

@jkrems I wanted to keep the behavior of the CJS module identical to what it is today, with the main export being a callable function, i.e., not exports.default = yargsParser. However, for ESM/Deno I wanted to export default yargsParser... I wasn't sure how to indicate to rollup that I wanted the export = yargsParser behavior, while at the same time using the desired syntax in the source files themselves.

Maybe I should open a ticket on the tsc rollup plugin I'm using?

[jkrems]

Yeah, it seem like if the module.exports object is set to the default export, then export = defaultExport would be the correct typings to emit. At least from what I understand about TS type definitions. Not sure how that would affect future ESM consumers (that use vscode code completion) in the presence of named exports etc. but that's not necessarily a problem for today..?

[evocateur]

Exciting!