Fix multiple escape parsing issues

[raskad]

This PR works on the escape parsing.

The PR changes the following:

• Fix a missing flag check where unicode property escapes would be parsed in non-unicode mode.
  • Add the unicode flag to some tests because of the above fix.
• Remove the unused Parser.max_backref field.
• Move the Term :: Assertion :: \b and Term :: Assertion :: \B parsing from atom escape parsing to term parsing.
• Add parsing for the case Term :: ExtendedAtom :: \ [lookahead = c].
• Add parsing for the case ClassEscape :: [~UnicodeMode] c ClassControlLetter.
• Add parsing for CharacterEscape :: [~UnicodeMode] LegacyOctalEscapeSequence.
• Fix errors thrown when CharacterEscape :: 0 [lookahead ∉ DecimalDigit], CharacterEscape :: HexEscapeSequence :: x HexDigit HexDigit or CharacterEscape :: RegExpUnicodeEscapeSequence where parsed unsuccessfully but IdentityEscape is still valid.
• Fix errors thrown when DecimalEscape parsed unsuccessfully but CharacterEscape os still valid.

These changes fix the following 262 tests:

test/language/literals/regexp/u-invalid-identity-escape.js
test/language/literals/regexp/named-groups/invalid-identity-escape-in-capture-u.js
test/annexB/language/literals/regexp/identity-escape.js
test/annexB/language/literals/regexp/legacy-octal-escape.js
test/annexB/language/literals/regexp/class-escape.js
test/annexB/built-ins/RegExp/incomplete_hex_unicode_escape.js
test/annexB/built-ins/RegExp/RegExp-decimal-escape-not-capturing.js
test/annexB/built-ins/RegExp/RegExp-trailing-escape.js
test/annexB/built-ins/RegExp/RegExp-decimal-escape-class-range.js
test/annexB/built-ins/RegExp/RegExp-leading-escape.js
test/built-ins/RegExp/unicode_restricted_identity_escape_alpha.js

[ridiculousfish]

Thank you! Gonna trust you on faithfully implementing the spec - I didn't check it, I'm not sure how to even read the latest editions 😬