Use prettier

[Deleted user (ghost)]

Fixes#18340

WIP -- Blocked on arijs/prettier-miscellaneous#29
Does not include actually applying prettier; most of the changes are due to the issue mentioned above.
You can test it out in a checkout by using gulp run prettier.
(We could use vanilla pretiter but then most of the changes would be moving every else branch up a line.)
Also includes a precommit hook to ensure we're actually running it.
We should also be sure to apply this after any huge PRs are in, such as #17269.

[Wesley Wigham (weswigham)]

I don't particularly mind running a second style tool; but we should probably also run tslint with --fix if we're going to start down the path of automatic fixes.

[Wesley Wigham (weswigham)]

Why not withFold<T>(name: string, action: () => T): () => T, making the body:

return()=>{if(fold.isTravis()){console.log(fold.start(name));}constresult=action();if(fold.isTravis()){console.log(fold.end(name));}returnresult;}

Allowing you to simply wrap whatever gulp task function you please? Should reduce nesting a bit, too (and clean up the diff).
Technically the fold won't work correctly for stream or promise return values... but w/e, it doesn't already and the higher-order definition looks a bit nicer at the use-site.

[Jessica Franco (Jessidhia)]

(Anecdote warning) In my experience tslint --fix was dangerous in that it could introduce invalid or unintended syntax, but the last time I encountered this problem was like a year ago so it's possible it is reliable now.

[Mohamed Hegazy (mhegazy)]

this has gone stale. closing for now.