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/kotlin/dev/hgh/jai/psi/JaiParserDefinition.kt

@@ -0,0 +1,36 @@
+package dev.hgh.jai.psi
+
+import com.intellij.lang.ASTNode
+import com.intellij.lang.ParserDefinition
+import com.intellij.lang.PsiParser
+import com.intellij.lexer.Lexer
+import com.intellij.openapi.project.Project
+import com.intellij.psi.FileViewProvider
+import com.intellij.psi.PsiElement
+import com.intellij.psi.PsiFile
+import com.intellij.psi.tree.IFileElementType
+import com.intellij.psi.tree.TokenSet
+import dev.hgh.jai.JaiLanguage
+import dev.hgh.jai.lexer.JaiLexer
+import dev.hgh.jai.lexer.JaiTokenTypes
+import dev.hgh.jai.parser.JaiParser
+
+class JaiParserDefinition : ParserDefinition {
+    override fun createLexer(project: Project?): Lexer = JaiLexer()
+
+    override fun createParser(project: Project?): PsiParser = JaiParser()
+
+    override fun getFileNodeType(): IFileElementType = FILE
+
+    override fun getCommentTokens(): TokenSet = JaiTokenTypes.COMMENTS
+
+    override fun getStringLiteralElements(): TokenSet = JaiTokenTypes.STRINGS
+
+    override fun createElement(node: ASTNode?): PsiElement = JaiTypes.Factory.createElement(node)
+
+    override fun createFile(viewProvider: FileViewProvider): PsiFile = JaiFile(viewProvider)
+
+    companion object {
+        val FILE = IFileElementType(JaiLanguage)
+    }
+}