Support .jsonc extension for all JSON files

[Dekkonot]

In #1144, we swapped our parser over to JSONC for all JSON projects. However, we did not support .jsonc as an extension for any of Rojo's files. This causes problems with linters and other niceties.

This PR fixes that! It adds support for:

• .meta.jsonc
• .project.jsonc
• .model.jsonc
• .jsonc

This turned out to be more complicated than anticipated because a lot of the support for default.project.json and .meta.json files were hardcoded.

To that end:

• ServeSession no longer hardcodes default.project.json and instead uses a function built into Project
• Several of the built-in project functions now support both default.project.json and default.project.jsonc
• Metadata support has been refactored so that the metadata itself handles loading it rather than individual middleware calling from_slice and applying it, so that .meta.jsonc support could be done in one place

[kennethloeffler]

Nice, I quite like the new helpers for meta files!

[ryanlua]

Looks like .model.jsonc isn't supported? I've been looking at this PR and see the code for .model.jsonc, but for some reason it doesn't want to sync.