new rule: no-mismatched-extensions

[boneskull]

This rule would determine the package type (ESM or CJS) via package.json, then check all the sources within to assert they are using the proper extensions.

e.g:

const isESM = isESMPkg(pkgJson);
let cjsExtensions: Set<string>;
let esmExtensions: Set<string>;
if (isESM) {
  cjsExtensions = new Set(['.cjs'])
  esmExtensions = new Set(['.js', '.mjs'])
} else {
  cjsExtensions = new Set(['.js', '.cjs'])
  esmExtensions = new Set(['.mjs'])
}