Add eslint-prettier

[LukeSheard]

With similar intentions to #224 this adds eslint-config-prettierand eslint-plugin-prettier to the eslint config, and adds a pre-commit hook which automatically formats all the javascript files for you.

• 4a534b7 adds the configuration to check for style errors
• 33592ba adds the pre-commit hook
• babb867 is an autoformat of the codebase to make sure this PR passes CI.

[texodus]

This is great, thanks for the PR!

As Perspective is a multi-language project (C++, Javascript and soon Python, plus non-languages like HTML and LESS), I believe it is important to impose at least some basic consistency across the codebase proper where applicable. We'll make a similar request to #224 when I have time to review the clang-format options in more detail, but for now I believe these overrides to prettier defaults are justified:

• bracketSpacing: false IMO the default is not consistent with the rest of the project - it is e.g. not the default for clang-format braced initializers, which they justify under Cpp11BracedListStyle here.
• tabWidth: 4 Again, simply for consistency with Added clang-format file #224 and the extent LESS, HTML, JSON, etc formatting.
• Worth also mentioning that we should probably have consistent printWidth, as I (and I assume any other developer) will not be switching IDEs when implementing changes across several Perspective languages. I do tend to agree however that Added clang-format file #224's printWidth is a bit on the conservative side, so this is probably fine at 200 for now.

Personally, I am disinterested in the details of code formatting outside of consistency - assuming you concur, would you mind amending this commit by adding the above 2 overrides to the prettier configuration in 4a534b7, and redoing babb867 to respect these changes?

[LukeSheard]

Very reasonable request. I was also unsure about printWidth being so large, but people are often strongly opinionated about these things so I just set it to something that was agreeable to sort out the other inconsistencies in the code. We can always update later.

I'll update the options now and re-run babb867.