[2.x] feat: case-insensitive tab completion (#8827)

Replace the custom JLineCompletion fallback with JLine 3's built-in
CASE_INSENSITIVE option on the LineReader, which handles case-insensitive
matching natively. Revert JLineCompletion.scala to its original state.

Fixes #2251

internal/util-complete/src/main/scala/sbt/internal/util/LineReader.scala

@@ -107,7 +107,12 @@ object LineReader {
     new LineReader {
       override def readLine(prompt: String, mask: Option[Char]): Option[String] = {
         val term = JLine3(terminal)
-        val reader = LineReaderBuilder.builder().terminal(term).completer(completer(parser)).build()
+        val reader = LineReaderBuilder
+          .builder()
+          .terminal(term)
+          .completer(completer(parser))
+          .option(JLineReader.Option.CASE_INSENSITIVE, true)
+          .build()
         try {
           inputrcFileContents.foreach { bytes =>
             InputRC.configure(

internal/util-complete/src/test/scala/sbt/internal/util/complete/JLineCompletionSpec.scala

@@ -0,0 +1,25 @@
+/*
+ * sbt
+ * Copyright 2023, Scala center
+ * Copyright 2011 - 2022, Lightbend, Inc.
+ * Copyright 2008 - 2010, Mark Harrah
+ * Licensed under Apache License 2.0 (see LICENSE)
+ */
+
+package sbt.internal.util
+package complete
+
+import org.scalacheck.*
+import org.scalacheck.Prop.*
+
+object JLineCompletionSpec extends Properties("JLineCompletion"):
+
+  property("case-insensitive completions are available at token start") =
+    val commands = Set("testOnly", "testQuick", "compile", "clean")
+    val parser = Parser.token(DefaultParsers.ID.examples(commands))
+    val completions = Parser.completions(parser, "", 1).get
+    val names = completions.map(_.append)
+    (names.contains("testOnly")) :| "testOnly is in initial completions" &&
+    (names.contains("compile")) :| "compile is in initial completions"
+
+end JLineCompletionSpec