ui: Adds multi syntax linting to the code editor

[johncowen]

This PR adds multi syntax linting capabilities to the code editor.

The linting/syntax javascript files are loaded on demand, so if you don't need a certain syntax, it won't be loaded. This gives us the freedom to add more syntaxes in the future with no downsides.

Currently we've included:

1. JSON: Syntax highlighting and linting
2. HCL: Syntax highlighting only
3. YAML: Syntax highlighting and linting

The look and feel is yet to be refined, although as it is functionally 'ready', I thought people might appreciate this in the upcoming release. It also might prompt users to give us any other syntaxes they might need including.

The syntax mode loader is a little 'out of the ordinary' as it tries to use the existing CodeMirror loadMode plugin to avoid reinventing the wheel.

The syntax highlighting colors themselves keep the colors we followed from the vault ui.

Fixes #4671 , #4258 and #4256

[johncowen]

P.S. I've had second thoughts about re-using the codemirror service.

I don't own it so I shouldn't be adding/mixing things into it. Hence the second best option is yet another service just for dealing with syntax/modes/linting. It's going to be a bit weird as it will be very specific to code mirror, but I think I prefer that. 🤔 Another option would be to extend the existing code mirror service instead and have like a second service('code-mirror/lintable') instead, at least then its clear that it has some extras.

In my mind, I don't think this would prevent this from going into a release soon if possible as it's more dev hygiene than anything.

[pearkes]

Looks good! I couldn't find anything surprising from my view. May be worth getting another set of 👀 but think this will be great.