feat!: y18n is now ESM first

[shadowspawn]

Stop shipping dual esm/cjs. Ship just esm and rely on require(esm) support in recent versions of node to support CJS users.

Following yargs moving from dual CJS/ESM to ESM-only: yargs/yargs#2451

Rework lint setup to closely follow setup in yargs/yarg: essentially switching from standardx to gts.

Lots of noise in "Files changed" due to linting changes, but can look at the first commit for the bulk of the esm changes.

Includes fix for module entry point:

• Fixes The files specified in module and exports are mismatched. #155
• Closes fix: entrypoint specified in module #156

[shadowspawn]

The rimraf interface changed from rimraf@3 to rimraf@6. The tests were failing because the cleanup of fr_FR.json was not working. Silly problem! After mucking about, I switched to using built-in plain fs.rmSync.

Next: sort out the linting... standardx not recognising esm syntax?

standardx: Use JavaScript Standard Style (tweaked by standardx) (https://github.com/standard/standardx)
  /Users/john/Documents/Sandpits/yargs/y18n/my-fork/test/y18n-test.mjs:10:46: Parsing error: Unexpected token import

[shadowspawn]

Two of the less trivial changes prompted by linter were:

Replace use of arguments with using rest ...args parameter.

Replace in _taggedLiteral:

- return this.__.apply(this, [str].concat([].slice.call(args, 1)))
+ return this.__(str, ...args.slice(1));

[shadowspawn]

I reworked README for esm.

Similar to yargs, code examples use y18n for the factory function and do not have a variable for the y18n instance created by the factory function. I explicitly talk about the factory function to distinguish it from the methods on a y18n instance. See how it scans for you.