Phase 3: Grammar-Kit parser, PSI and ParserDefinition

Adds src/main/grammar/Jai.bnf, the generated parser/PSI in src/main/gen, a
JaiParserDefinition, and the Tier 3 corpus parse gate (406/714 files clean).

Two things were not obvious:

- Grammar-Kit mints its own token instances from the .bnf 'tokens' block, which
  are different objects from the ones JaiLexer emits, so every rule silently
  failed to match. Fixed with tokenTypeFactory -> JaiTokenTypes.byName.
- A backslash inside an identifier is a continuation in the compiler's lexer
  (Jai_Lexer/module.jai:444): 'left\_margin' is one identifier. JaiLexer now
  does the same; the Tier 0 round-trip still holds because the token span
  covers the backslash and the skipped spaces.

Constructs that BNF alone cannot express live in JaiParserUtil: directive-name
tests (#ident is one token), '==' before '{' for the switch form, procedure
header vs parenthesised expression, and the declaration lookahead.

src/test/testData/parsing/SimpleDeclarations.txt

@@ -0,0 +1,41 @@
+Jai File(0,49)
+  JaiStatementImpl(STATEMENT)(0,17)
+    JaiDeclarationImpl(DECLARATION)(0,17)
+      JaiDeclNamesImpl(DECL_NAMES)(0,1)
+        PsiElement(Jai:IDENT)('a')(0,1)
+      PsiElement(Jai::)(':')(2,3)
+      JaiRefExprImpl(REF_EXPR)(4,9)
+        PsiElement(Jai:IDENT)('float')(4,9)
+      PsiElement(Jai:=)('=')(10,11)
+      JaiInitializerImpl(INITIALIZER)(12,16)
+        JaiLiteralExprImpl(LITERAL_EXPR)(12,16)
+          PsiElement(Jai:NUMBER)('37.0')(12,16)
+      PsiElement(Jai:;)(';')(16,17)
+  JaiStatementImpl(STATEMENT)(18,26)
+    JaiDeclarationImpl(DECLARATION)(18,26)
+      JaiDeclNamesImpl(DECL_NAMES)(18,19)
+        PsiElement(Jai:IDENT)('b')(18,19)
+      PsiElement(Jai::=)(':=')(20,22)
+      JaiInitializerImpl(INITIALIZER)(23,25)
+        JaiLiteralExprImpl(LITERAL_EXPR)(23,25)
+          PsiElement(Jai:NUMBER)('10')(23,25)
+      PsiElement(Jai:;)(';')(25,26)
+  JaiStatementImpl(STATEMENT)(27,35)
+    JaiDeclarationImpl(DECLARATION)(27,35)
+      JaiDeclNamesImpl(DECL_NAMES)(27,28)
+        PsiElement(Jai:IDENT)('c')(27,28)
+      PsiElement(Jai:::)('::')(29,31)
+      JaiInitializerImpl(INITIALIZER)(32,34)
+        JaiLiteralExprImpl(LITERAL_EXPR)(32,34)
+          PsiElement(Jai:NUMBER)('42')(32,34)
+      PsiElement(Jai:;)(';')(34,35)
+  JaiStatementImpl(STATEMENT)(36,49)
+    JaiReturnStatementImpl(RETURN_STATEMENT)(36,49)
+      PsiElement(Jai:return)('return')(36,42)
+      JaiAddExprImpl(ADD_EXPR)(43,48)
+        JaiRefExprImpl(REF_EXPR)(43,44)
+          PsiElement(Jai:IDENT)('a')(43,44)
+        PsiElement(Jai:+)('+')(45,46)
+        JaiRefExprImpl(REF_EXPR)(47,48)
+          PsiElement(Jai:IDENT)('b')(47,48)
+      PsiElement(Jai:;)(';')(48,49)