Fix #4645, Add an extension to _mocha for people with loaders

[FossPrime]

Description of the Change

Alternate Designs

• Moving mocha to mocha.js: Docs hard reference mocha without extension.
• Making a copy of _mocha to mocha.cjs: This is uncommon. Though it's strongly recommended by Node and others when you transpile to CJS from a native ES Module. https://redfin.engineering/node-modules-at-war-why-commonjs-and-es-modules-cant-get-along-9617135eeca1
• Loading mocha with a PassThrough stream in mocha.js: That a little exotic and depends on stdio
• We could make a mjs wrapper, but that seems unnecessary right now... it is a workaround however

// mocha-4645.mjs
import cli from './node_modules/mocha/lib/cli/index.js'
cli.main()

Why should this be in core?

It cleanly allows for TS and ESM support, using loaders. A recurring long standing request.

Benefits

ts-node/esm and other node loaders will work as expected. As well as supporting custom loaders, custom loaders for logging, performance, etc.

Possible Drawbacks

If something resolves the symlinks in .bin/ manually AND looks for a /mocha$/ regex... this would break their code. I don't know any reason to do that and they would using multiple levels of non public APIs, so I'm dismissing this.

Applicable issues

#4645

[coveralls]

Coverage remained the same at 94.344% when pulling 62d8708 on rayfoss:node-loader-bugfix into a93d759 on mochajs:master.