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/main/gen/dev/hgh/jai/psi/impl/JaiPushContextStatementImpl.java

@@ -0,0 +1,42 @@
+// This is a generated file. Not intended for manual editing.
+package dev.hgh.jai.psi.impl;
+
+import java.util.List;
+import org.jetbrains.annotations.*;
+import com.intellij.lang.ASTNode;
+import com.intellij.psi.PsiElement;
+import com.intellij.psi.PsiElementVisitor;
+import com.intellij.psi.util.PsiTreeUtil;
+import static dev.hgh.jai.psi.JaiTypes.*;
+import com.intellij.extapi.psi.ASTWrapperPsiElement;
+import dev.hgh.jai.psi.*;
+
+public class JaiPushContextStatementImpl extends ASTWrapperPsiElement implements JaiPushContextStatement {
+
+  public JaiPushContextStatementImpl(@NotNull ASTNode node) {
+    super(node);
+  }
+
+  public void accept(@NotNull JaiVisitor visitor) {
+    visitor.visitPushContextStatement(this);
+  }
+
+  @Override
+  public void accept(@NotNull PsiElementVisitor visitor) {
+    if (visitor instanceof JaiVisitor) accept((JaiVisitor)visitor);
+    else super.accept(visitor);
+  }
+
+  @Override
+  @NotNull
+  public JaiBlock getBlock() {
+    return findNotNullChildByClass(JaiBlock.class);
+  }
+
+  @Override
+  @NotNull
+  public JaiExpr getExpr() {
+    return findNotNullChildByClass(JaiExpr.class);
+  }
+
+}