feat(core): transition from native @namespace (conflict)

[idoros]

This PR adds an alternative to @namespace in order to remove conflict with native CSS.

Tasks

1. move code to st-namespace feature
2. add @st-namespace atrule
3. namespace validation:
   • valid (ident-like):
     @st-namespace "dropdown";
@st-namespace 'dropdown';
@st-namespace "group-card";
@st-namespace "MixCapitalization";
   • invalid:
     @st-namespace "http://www.w3.org/1999/xhtml"; <- collect just ident-like
     @st-namespace prefix "http://www.site.com/foo"; <- no prefix
     @st-namespace url(https://www.site.com); <- no url
4. collect from @namespace for backward compatibility (just when matches validation)
5. remove native @namespace only if a namespace was generated from it
6. prefer @st-namespace
7. change language server to complete @st-namespace instead and add it as a known at-rule
8. @namespace to @st-namespace code mod

• backport to v4.x - feat(core) transition from native @namespace - v4 #2641
• update documentation to refer to @st-namespace only (maybe a small section about previous behavior)
• add issue to stop collecting stylesheet namespace from @namespace in the next major

[barak007]

final stuff