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/test/kotlin/dev/hgh/jai/editor/JaiEditorSupportTest.kt

@@ -0,0 +1,60 @@
+package dev.hgh.jai.editor
+
+import com.intellij.codeInsight.highlighting.BraceMatchingUtil
+import com.intellij.lang.LanguageBraceMatching
+import com.intellij.lang.LanguageCommenters
+import com.intellij.openapi.editor.ex.EditorEx
+import com.intellij.testFramework.fixtures.BasePlatformTestCase
+import dev.hgh.jai.JaiFileType
+import dev.hgh.jai.JaiLanguage
+
+class JaiEditorSupportTest : BasePlatformTestCase() {
+    fun testCommenterIsRegistered() {
+        val commenter = LanguageCommenters.INSTANCE.forLanguage(JaiLanguage)
+        assertTrue("expected JaiCommenter, got $commenter", commenter is JaiCommenter)
+        assertEquals("//", commenter.lineCommentPrefix)
+        assertEquals("/*", commenter.blockCommentPrefix)
+        assertEquals("*/", commenter.blockCommentSuffix)
+    }
+
+    fun testBraceMatcherIsRegistered() {
+        val matcher = LanguageBraceMatching.INSTANCE.forLanguage(JaiLanguage)
+        assertTrue("expected JaiBraceMatcher, got $matcher", matcher is JaiBraceMatcher)
+    }
+
+    fun testMatchesOrdinaryBraces() {
+        assertMatchForward("main :: () |{ x := 1; }", "main :: () { x := 1; |}")
+    }
+
+    fun testMatchesStructLiteralBrace() {
+        // `.{` is one token; it still has to pair with the ordinary closing brace.
+        assertMatchForward("p := Vector2.|{ 1, 2 };", "p := Vector2.{ 1, 2 |};")
+    }
+
+    fun testMatchesArrayLiteralBracket() {
+        assertMatchForward("names := .|[ \"a\", \"b\" ];", "names := .[ \"a\", \"b\" |];")
+    }
+
+    /**
+     * Both arguments are the same text with a `|` marking the caret; the first marks the
+     * opening brace, the second the offset the matcher must jump to.
+     */
+    private fun assertMatchForward(
+        withOpen: String,
+        withClose: String,
+    ) {
+        val open = withOpen.indexOf('|')
+        val close = withClose.indexOf('|')
+        val text = withOpen.replace("|", "")
+        assertEquals("the two markers must describe the same text", text, withClose.replace("|", ""))
+
+        myFixture.configureByText("braces.jai", text)
+        val editor = myFixture.editor as EditorEx
+        val iterator = editor.highlighter.createIterator(open)
+        assertTrue(
+            "no forward match found for the brace at $open in $withOpen",
+            BraceMatchingUtil.matchBrace(editor.document.charsSequence, JaiFileType, iterator, true),
+        )
+        assertEquals(close, iterator.start)
+    }
+}