Migrating to ECMAScript modules

[Janther]

I know this PR looks very daunting.

Here are the main changes:

• elsint, jest, and webpack configurations were changed accordingly
• the build target file is now dist/standalone.cjs instead of dist/standalone.js
• all require were switched to import
• all module.exports were switched to export default or export individual functions
• all __dirname were changed to meta.import and then extracted from that value
• some functions used the variable doc that was changed to document since now the import exposes prettier.doc directly
• the generateindexes script had to be removed since dir-to-object doesn't support esm
• the testing configuration scripts were adapted based on the changes that prettier made to their tests when they migrated
• standalone tests use a commonjs environment and normal tests use an esm environment so we need esmock or proxyquire depending on the situation

Caveats:

• if you are still using prettier 2 or a commonjs development environment, then you have to use the compiled version of the plugin dist/standalone.cjs (this is done automatically by node and conditional exports), unit tests for prettier 2 had to be disabled for this reason

[fvictorio]

Hey @Janther, I won't have time to review this but I did try it locally with both Prettier v2 and v3 (using verdaccio, which I recommend for scenarios like this one).

Prettier v3 works fine, but v2 doesn't because the dist/standalone.cjs file wasn't included in the files array. I think the easiest thing to do is to just include the full directory, which I did in the last commit. But if you prefer explicitly listing each file, feel free to revert that commit and use that approach.

In any case, I'm pre-approving it.

[Janther]

My bad. I'll push a commit making sure webpack cleans the folder before building to avoid publishing random stuff