Consider changing base scope to `source.less.css`

[radium-v]

Since the package ecosystem is fragmented, I'm considering changing the base scope and the scopes of CSS-specific matches to improve compatibility with other packages and color schemes already in the wild.

I made this spreadsheet to highlight the differences between syntax highlighting packages for Less and CSS: https://docs.google.com/spreadsheets/d/18vckSzvwBYICZZsmEz6xV-WLQv4jlTJ4WvAf4pbjOe0/edit?usp=sharing

I need to do more research on other related packages, like build systems and snippets that rely on specific base scopes.

[radium-v]

VS Code is converting the scope to source.css.less with a transform:
https://github.com/microsoft/vscode/blob/05cd2640ec8a106a4ee99cb38e6ee34fbec04f11/extensions/less/build/update-grammar.js#L11C17-L11C17