Document PhET software design patterns related to TypeScript.

[pixelzoom]

TS is going to result in revisions to existing PhET design patterns (e.g. options, enumeration), and will likely result in some new patterns. These TS patterns should eventually be documented, as has been done for JS patterns in https://github.com/phetsims/phet-info/blob/master/doc/phet-software-design-patterns.md. It's unclear whether they should be documented in that same .md file, in a new .md file,... In any case, keep in mind that the JS patterns will be around for a while (maybe forever).

[samreid]

@chrisklus and I discussed the need for a "conventions" document, which is different than a "design patterns" document and possibly also different than the "code review" doc. I added a first draft from our collaboration above.

[zepumph]

I think it would be worth noting phetsims/ratio-and-proportion#405, as most of these felt like "conventions" as opposed to patterns or syntax/code-style questions.

[samreid]

I added a few more entries to https://github.com/phetsims/phet-info/blob/master/doc/typescript-conventions.md

[samreid]

We have improved and elaborated on our TypeScript documentation in other issues, closing.