Add support for language specific toggles

[Noorts]

Allow adding toggles to the configuration which are only enabled for files in certain languages.

The following is an example configuration just to get the point across. Of course with an optimal and valid JSON structure (which is backwards compatible with the current structure).

{
    "default": [
        ["byte", "short", "int", "long", "float", "double"],
        ["String", "Character"],
    ],
    "java": [
        ["public", "private", "protected"],
        ["class", "interface"],
        ["extends", "implements"],
        ["import", "export"],
    ],
    "js, jsx, ts, tsx": [
        ["const", "let", "var"],
    ]
}

• Determine format (also JSON vs YAML)
• Determine whether language specific toggles inherit from the generic toggles (also deal with overwrites)
• Dig into underlying plugin storage (avoid erasing existing toggles upon update; might want to save the serialized representation instead of the current deserialized one)
• Implement the feature

[Noorts]

The (now 5 years out-of-date) Shifter plugin does have support for this. They seem to be using a custom toggle definition format for it.

The VSCode Toggler has supported this since 0.2.0, but uses VSCode's language specific settings feature for it.

[Noorts]

Might want to consider switching from JSON to YAML as well. This could simplify the parsing (e.g., need for boundary characters).

[Noorts]

The key problem to consider will be ensuring that current users of the plugin do not lose their toggles when they update. Combining this with the code debt described in #13 might be wise.