Fix calls to string case api (#1620)

* Enable IntelliJ inspection for calls to `String.toLowerCase()` and
`String.toUpperCase()`
* Enable error prone check
* Fix all issues

pkl-parser/src/main/java/org/pkl/parser/Token.java

@@ -15,6 +15,8 @@
  */
 package org.pkl.parser;
 
+import java.util.Locale;
+
 public enum Token {
   ABSTRACT,
   AMENDS,
@@ -233,6 +235,6 @@ public enum Token {
     if (this == UNDERSCORE) {
       return "_";
     }
-    return name().toLowerCase();
+    return name().toLowerCase(Locale.ROOT);
   }
 }