build: update typescript, eslint, prettier and more

[a-b-r-o-w-n]

• Typescript 3.9
• Prettier 2.0
• Eslint 7

closes #2570

[beyackle]

This is a huge set of file diffs to look through - am I correct that the biggest changes here are to put parentheses around function arguments (e.g. (x) => x+2) and to alphabetize props in components everywhere? I support the former, but I don't know if I agree with the latter. It does make props easier to find in a long list, but it means they can't be grouped functionally - e.g. if I wanted to put all of the onEvent props at the end of a component as a convention.

[a-b-r-o-w-n]

@beyackle You are correct. You are also in luck, the alphabetized props already groups event handlers on* together at the end!

edit: the parens for arrow functions is now a prettier default. The reasoning is that it makes it easier to 1) add additional parameters and 2) add param typing.

Also, the vast majority of changes to src files were eslint --fix changes. There were just a few things that the new ts compiler started complaining about, and I tried to isolate those changes in their own commits.

[a-b-r-o-w-n]

@beyackle I'm open to discuss new / changes to eslint rules. With respect to sorting props, I like that you don't have to think about it -- every prop has its place!

[beyackle]

Giving it some more thought, I think I agree with the alphabetized props. It's one fewer way code can be changed without changing its meaning. If we decide it's too annoying, we can revert that part.

[beyackle]

The automated changes look pretty good, but there are a few small things I think still need to be changed.

[beyackle]

LGTM. 🚢