Update TypeScript to 3.8.3 and Prettier to 2.0.4

[NaridaL]

I want to use ?. in my other pull request.

[coveralls]

Coverage remained the same at 93.489% when pulling cf9d0f3 on NaridaL:typescript3.8 into 7f4df9b on mobxjs:master.

[NaridaL]

@xaviergonz gonna tag you because you were active here recently 😄

[mweststrate]

Please do make sure you don't commit unrelated files. Your prettier configs seems to deviate from what is used in this repo, making this PR much larger than it should be. Feel free to commit a .prettierrc file to fix this if this avoids this repo conflicting with your IDE defaults.

[mweststrate]

This seems to be the result of using a different prettier config or version. Please align your local settings with this repo, or don't stage the unrelated files, or creating a PR that locks the config and prettier version would be even more awsome!

[mweststrate]

No unrelated files touched by this PR

[NaridaL]

Yeah sorry, this PR should be called "update typescript and prettier". Prettier 2 supports the TS 3.8 features.

Prettier 2 contains some breaking changes concerning default options and formatting, hence the number of changes. See also https://prettier.io/blog/2020/03/21/2.0.0.html

As the new defaults make sense I thought I'd just reformat everything although I guess we could manually change the config so it doesn't change much.

[mweststrate]

@NaridaL that makes sense, thanks for the explanation! The new dangling commas everywhere, is that a change in the prettier defaults? If it isn't, the .prettierrc should be updated accordingly

[mweststrate]

Answer: yes, it is the new default

[NaridaL]

@mweststrate do you mind hitting merge? Or you can add me as maintainer here too.

Also, I'd appreciate you taking a look at #245 😄

[mweststrate]

@NaridaL sorry, very swamped atm! I'll send an invite.