feat: add @namespace validation to no-invalid-at-rule-placement rule

[TKDev7]

Prerequisites checklist

• I have read the contributing guidelines.

What is the purpose of this pull request?

This pull request implements and enforces the correct placement of the @namespace at-rule in CSS, according to the specification: @namespace must be placed before all other rules, except @charset and @import.

What changes did you make? (Give an overview)

• Updated the no-invalid-at-rule-placement rule to check for the correct placement of @namespace.
• Added tests to verify correct and incorrect placements of @namespace and other at-rules.
• Updated documentation to reflect the new rule behavior and provide usage examples.

Related Issues

Is there anything you'd like reviewers to focus on?

[snitin315]

Hi @TKDev7, are you still working on this?

[TKDev7]

@snitin315 #171 needs to be merged first.

[snitin315]

Done

[lumirlumir]

Hi, @TKDev7

Could you please add appropriate descriptions to the PR template?

[nzakas]

The code looks good. Just want to clarify some things in the docs.

[nzakas]

LGTM. Thanks!