Allows to disable CSS variables suggestions

Hello,

Currently, this extensions only supports CSS variables suggestions from the current file #156

There are others extensions, that specialise in providing CSS variables suggestions like:

• https://marketplace.visualstudio.com/items?itemName=vunguyentuan.vscode-css-variables
• https://marketplace.visualstudio.com/items?itemName=phoenisx.cssvar

The problems is that the suggestions from the different extensions are merged with the one from vscode-css-languageservice:

This leads to duplicate suggestions and also makes the navigation more difficult. For example, if I try to go to the definition of a variable, I don't go there directly I have a "preview popup" opening asking me to choose which one I want.

https://github.com/microsoft/vscode-css-languageservice/assets/4760796/cdf00dc1-8def-49e4-b6e8-278e4b307024

It would be nice if vscode-css-languageservice had a setting to disable their CSS variables suggestions so others extensions can take over.

I think this is important to allows the users to use other extensions for CSS variables completion because there are a lot of different situation to supports:

• Allows to support completion across files #156
• Supports completion in the different CSS languages like SCSS, SASS, LESS, etc.
• Support completion from an external CSS file
• etc.

This is not possible / right, to expect from vscode-css-languageservice to supports all these scenarios but I think it is important to not "degrade" the experience of the other extensions who specialise in CSS variables completions.

Related issues: https://github.com/willofindie/vscode-cssvar/issues/66 https://github.com/willofindie/vscode-cssvar/issues/83

The issue does not name files or tests; start by locating the CSS variable completion entry point in vscode-css-languageservice and tracing how completion settings are handled. Done means providing a setting that suppresses the service's CSS variable suggestions while leaving other completions available for external extensions.