Adding values and functions to custom data

Adding custom values and functions to custom data would make it easier and more maintainable to have a richer editor experience for these tokens.

For PostCSS specifically it is not possible to create a general purpose language extension that covers all possible custom additions. Every user's collection of plugins is a bit different.

Plugin authors could also publish relevant custom data for their PostCSS plugins.

This can also act as an escape hatch for newer language features like oklch(0 0 0).

.foo {
  color: my-custom-color-ident;
  color: my-custom-color-function(0 0 0);
}

The issue names no files, tests, or entry points, so first locate how the language service represents and loads custom data. Define how custom identifiers and functions should be supplied for CSS and PostCSS extensions, then add coverage showing that the examples in the issue are recognized.