feat: migrate file extension from .as to .affine for GitHub Linguist recognition (#15)

The .as extension conflicts with ActionScript in GitHub Linguist, causing all
AffineScript source files to be misclassified as ActionScript/JavaScript. This
migration to .affine resolves the conflict and enables proper language detection.

Changes:
- Rename all 153 .as source files to .affine across conformance/, examples/,
  stdlib/, test/, and tests/ directories
- Add linguist-language=AffineScript directive to .gitattributes for .affine
- Update tree-sitter grammar file-types from "as" to "affine"
- Update VSCode extension to register .affine instead of .as
- Update all documentation references (28+ files) to use .affine
- Update test fixture references in test_e2e.ml

https://claude.ai/code/session_015S2ndRbwuJPpjkv54BpVDw

Co-authored-by: Claude <noreply@anthropic.com>

Author: JoshuaJewell