feat: import and use syntax support

[SchoolyB]

Summary

Grayscale supports a combined import and use statement (STANDARD.md §8.3) that imports a module and brings its members into scope in a single line. The LSP does not recognize this syntax.

Example

import and use @arrays
import and use @arrays, @strings

Equivalent to:

import @arrays
using arrays

Scope

• Recognize import and use as valid syntax (no false diagnostics)
• Treat the imported module as both imported and "used" for completion purposes
• Add hover docs for the use keyword explaining its role in import and use

[SchoolyB]

Implemented in 5f1d548. The syntax was already accepted — diagnostics come from grayc — but module completion had a bug.

The completion trigger could never match this form. It tested /\bimport\s+@\w*$/ against the line prefix, which requires @ to follow import directly. So import and use @ offered no modules at all. The same regex also missed the comma continuation of the plain form: typing import @io, @ma stopped suggesting after the first module.

Replaced with a trigger anchored to the start of the line that accepts the optional and use and any number of comma-separated modules. Verified against the negative cases too — import c"std does not trigger it, and neither does println("import @"), which the old unanchored \b pattern would have matched inside a string literal.

Also updated on the docs side: and added as a keyword, and the import, use, and using entries rewritten to cover the combined statement, file-scope vs function-scope using, and the local/C import forms.