[JS IR] Namer improvements

- Compute and store local names locally when translating a body.
  It is a step towards separate JS generation and hopefully reduces memory usage.

- Use stable mangled names for member names. Needed for separate JS generation.

- Add `abstract class IrNamerBase` with just 3 abstract methods to simplify
  creating new IrNamer implementations.

- Fix O(N^2) of findFreshName when it is called wtih the same name suggestion a lot of times.

- Refactor NameTables initialisation: factor out some functions and use descriptive names.

- Use StringBuilder in sanitizeName

js/js.frontend/src/org/jetbrains/kotlin/js/naming/NameSuggestion.kt

@@ -399,7 +399,6 @@ fun Char.isES5IdentifierPart(): Boolean {
                 Character.CONNECTOR_PUNCTUATION -> true
                 else -> false
             } ||
-            // Nl which is missing in Character.isLetter, but present in UnicodeLetter in spec
             this == '\u200C' ||   // Zero-width non-joiner
             this == '\u200D'      // Zero-width joiner
 }