feat: add vala-lint per-file highlighting support

[Tbusk]

Description

Add support for per-file linter support with vala-lint which shows errors on each line with the message and severity highlighted.

Possible Solutions

Either create a new plugin with this capability or add in an opt-in toggle in the settings with this feature.

Additional Info

This can be created using ExternalAnnotator and a json output of vala-lint that is parsed.

The json output contains everything that is needed in order to show this output. Here is a quickly thrown-together sample:

[Tbusk]

A prototype is available in branch feature/vala-lint-highlighting