Phase 2: commenter and brace matcher

Brace matching pairs the Jai-specific single-token openers .{ and .[ with the
ordinary } and ]; verified headlessly through the editor highlighter.

src/main/kotlin/dev/hgh/jai/editor/JaiEditorSupport.kt

@@ -0,0 +1,52 @@
+package dev.hgh.jai.editor
+
+import com.intellij.lang.BracePair
+import com.intellij.lang.Commenter
+import com.intellij.lang.PairedBraceMatcher
+import com.intellij.psi.PsiFile
+import com.intellij.psi.tree.IElementType
+import dev.hgh.jai.lexer.JaiTokenTypes
+
+/** `//` line comments and nesting `/* */` block comments (language reference §2). */
+class JaiCommenter : Commenter {
+    override fun getLineCommentPrefix(): String = "//"
+
+    override fun getBlockCommentPrefix(): String = "/*"
+
+    override fun getBlockCommentSuffix(): String = "*/"
+
+    // Jai block comments nest, so a commented-out region can be commented again.
+    override fun getCommentedBlockCommentPrefix(): String = "/*"
+
+    override fun getCommentedBlockCommentSuffix(): String = "*/"
+}
+
+/**
+ * Brace matching. Note the two Jai-specific openers: `.{` starts a struct literal and
+ * `.[` starts an array literal (language reference §14.4) — they are single tokens, and
+ * close with an ordinary `}` / `]`.
+ */
+class JaiBraceMatcher : PairedBraceMatcher {
+    override fun getPairs(): Array<BracePair> = PAIRS
+
+    override fun isPairedBracesAllowedBeforeType(
+        lbraceType: IElementType,
+        next: IElementType?,
+    ): Boolean = true
+
+    override fun getCodeConstructStart(
+        file: PsiFile?,
+        openingBraceOffset: Int,
+    ): Int = openingBraceOffset
+
+    companion object {
+        private val PAIRS =
+            arrayOf(
+                BracePair(JaiTokenTypes.LBRACE, JaiTokenTypes.RBRACE, true),
+                BracePair(JaiTokenTypes.LPAREN, JaiTokenTypes.RPAREN, false),
+                BracePair(JaiTokenTypes.LBRACKET, JaiTokenTypes.RBRACKET, false),
+                BracePair(JaiTokenTypes.BEGIN_STRUCT_LITERAL, JaiTokenTypes.RBRACE, false),
+                BracePair(JaiTokenTypes.BEGIN_ARRAY_LITERAL, JaiTokenTypes.RBRACKET, false),
+            )
+    }
+}