Make vscode-extension-schema honour default values

[fendor]

Follow up to #3917, where the command hls vscode-extension-schema did not to honour the default values of the plugin description.

The first commit of this PR fixes that shortcoming.
The second commit unifies the plugin config generation. I am not sure whether that's a good idea, as I feel it obfuscates the code more. I am leaving it in for discussion, but I am completely open to dropping the second commit.

[fendor]

This is likely easier to review once #3922 is merged

[fendor]

I decided to drop the unification of the code paths, as the subtle differences made it unnecessarily difficult.

[joyfulmantis]

LGTM

[michaelpj]

has conflicts

[soulomoon]

           "haskell.plugin.semanticTokens.globalOn": {
      -        "default": true,
      +        "default": false,
               "description": "Enables semanticTokens plugin",
               "scope": "resource",
               "type": "boolean"

the newly added semanticTokens should be false by default.