Map @lsp.mod.readonly to constant

[tkrause]

Is your feature request related to a problem? Please describe. Highlighting does not work for some LSPs with constants when semantic tokens are enabled. This is due to a missing @lsp.mod.readonly mapping and can be seen with gopls and likely others.

Describe the solution you'd like Add support for ["@lsp.mod.readonly"] = { link = "Constant" }

Describe alternatives you've considered I'm using the above in my custom_overrides which solves my issue but it would be nice if this was just built-in.