Add builtin character class atoms to the DSL tree

[Hamish Knight]

For the following escape builtins:

• graphemeCluster
• whitespace
• decimalDigit
• horizontalWhitespace
• newlineSequence
• verticalTab
• wordCharacter

We currently form DSL character classes using their unconverted AST atoms. We ought to have DSL representations for them, perhaps under an e.g BuiltinCharacterClass type.

[Hamish Knight]

This could also be done alongside migrating them off consumers and onto dedicated instructions in byte code gen

[Hamish Knight]

Lily has a PR for this #547

[Lily]

Fixed in #547