Provide a way to override non-boolean server capabilities in the config

[jwortmann]

Is your feature request related to a problem? Please describe.

JETLS includes the space character in the completion trigger characters, and that causes the completion popup to open whenever pressing space while typing, which I don't like.

Currently I can use

"disabled_capabilities": {
    "completionProvider.triggerCharacters": true,
}

to disable all trigger characters. But I'd rather like to disable only the space character, and keep the other trigger characters active.

Describe the solution you'd like

Perhaps a new config option

"capability_overrides": {
    "completionProvider.triggerCharacters": ["@", "\\", ":"]
}

that would override the given (sub-)key in the server capabilities.

So this would be a more flexible variant of disabled_capabilities. Other (non-boolean) server capabilities where this could be useful are signatureHelpProvider.triggerCharacters and documentOnTypeFormattingProvider.moreTriggerCharacter.

Furthermore, by convention the value null could be used to "unset" the given key and all sub-keys of that key, for example

"capability_overrides": {
    "completionProvider.triggerCharacters": null
}

would be equivalent to

"disabled_capabilities": {
    "completionProvider.triggerCharacters": true,
}

That would allow to replace and deprecate disabled_capabilities, if we want to do that. Just an idea.