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.

tree-sitter-storybook/src/grammar.json

@@ -1,4 +1,5 @@
 {
+  "$schema": "https://tree-sitter.github.io/tree-sitter/assets/schemas/grammar.schema.json",
   "name": "storybook",
   "word": "identifier",
   "rules": {
@@ -1294,11 +1295,11 @@
               "members": [
                 {
                   "type": "STRING",
-                  "value": "extends"
+                  "value": "modifies"
                 },
                 {
                   "type": "FIELD",
-                  "name": "extends",
+                  "name": "modifies",
                   "content": {
                     "type": "SYMBOL",
                     "name": "identifier"
@@ -3086,6 +3087,6 @@
   "precedences": [],
   "externals": [],
   "inline": [],
-  "supertypes": []
-}
-
+  "supertypes": [],
+  "reserved": {}
+}