use mocha as a peer dependency

[jehy]

Using mocha as a peer dependency does not break old node (developer will only need to specify mocha version they need in package.json) but allows to use latest version of mocha as requested here #6

[ljharb]

If we're going to support multiple mocha major versions, we need to test on each one.

[ljharb]

* is never an acceptable value on the RHS of package.json; we should explicitly list each major version we support, ie ^3 || ^4 || ^5 || ^6 || ^7 etc.

[jehy]

What about "3+" then? Seems like a pretty legit semver value to me.

[ljharb]

>= 3 is valid, but has the same problem that if mocha 8, or 12, or 25 comes out, and doesn't work with us, we'll be falsely claiming it works.

Only explicitly tested mocha versions should be allowed in peer deps (which are a hard install requirement in npm ls, npm 7+, yarn list, and yarn 2+)