Document --check

[thheller]

--check is very useful for finding missing externs in any build.

CLOSURE-WARNING: shadow/cljs/npm/cli.cljs[7:12] (compiled to shadow/cljs/npm/cli.js[8:79])
	Property statSync never defined on shadow.npm.fs

CLOSURE-WARNING: shadow/cljs/npm/cli.cljs[53:13] (compiled to shadow/cljs/npm/cli.js[97:165])
	Property existsSync never defined on shadow.npm.fs

CLOSURE-WARNING: shadow/cljs/npm/cli.cljs[54:4] (compiled to shadow/cljs/npm/cli.js[99:143])
	Property mkdirSync never defined on shadow.npm.fs

Used it to find all the :externs required for the shadow.cljs.npm.cli script.

Also useful since it does type checking but given that CLJS is mostly untyped it only finds errors in the Closure lib or when using typed :externs. Given that these are mostly incorrect --check may actually just confuse more than help.

Also should probably talk about how people are doing :externs wrong.

[thheller]

Refactored the warning code so it now also generates pretty warnings for --check.

The message isn't 100% ideal and column information is missing but it is better than before.

[thheller]

Turns out that check is super unreliable in many cases. It's still in there but might report so many false positives that it might confuse more than actually help.

Should rely on better externs inference instead.