feat(lang): complete extends to modifies keyword migration

This commit completes the migration started in the previous commit,
updating all remaining files:

- Lexer: Changed token from Extends to Modifies
- Parser: Updated lalrpop grammar rules and AST field names
- AST: Renamed Schedule.extends field to modifies
- Grammar: Updated tree-sitter grammar.js
- Tree-sitter: Regenerated parser.c and node-types.json
- Examples: Updated baker-family work schedules
- Tests: Updated schedule composition tests and corpus
- Docs: Updated all reference documentation and tutorials
- Validation: Updated error messages and validation logic
- Package: Bumped version to 0.3.1 in all package manifests

All 554 tests pass.

Cargo.toml

@@ -1,6 +1,10 @@
+[workspace]
+members = [".", "xtask"]
+exclude = ["tree-sitter-storybook"]
+
 [package]
 name = "storybook"
-version = "0.2.0"
+version = "0.3.1"
 edition = "2021"
 
 [lib]