Allow comments in global.json

[dorssel]

Description:
Strip c-style comments from global.json before parsing.
I recommend https://www.npmjs.com/package/strip-json-comments, which helps keeping positional error reporting accurate.

Justification:
dotnet itself allows comments in global.json, so when I wanted to setup this action for an existing project it failed without modifying global.json. As an alternative, I had to use with: dotnet-version, which goes against the 'single source of truth' principle.

This is not to start a flame war; I am well aware that the standard JSON does not allow comments. And for data-interchange I agree. But for configuration it is actually quite handy. JSON5 allows it, and so does tsconfig.json (this very project is TypeScript).

Are you willing to submit a PR?
Sure.

[xt0rted]

https://github.com/json5/json5 supports this and should be a drop in replacement for the built-in JSON object.

[dorssel]

@xt0rted
I am a bit reluctant to simply replace the entire JSON library. It should work, but JSON5 does a lot more than just allow comments.

Instead, I implemented #257, which is really a one-liner doing exactly as the proposal requested: remove comments before parsing global.json. Nothing more than that.

[dorssel]

Abandoned #257. See comments there...