[editorial] Unconditionally use LookaheadAssertion in Assertion

[nicolo-ribaudo]

This PR tries to simplify the Annex B RegExp Assertion production, by removing the branching on the UnicodeMode parameter. The two branches were effectively the same, just with the UnicodeMode parameter hard-coded to either enabled or disabled depending on whether it was enabled or disabled.

Additionally, it renames QuantifiableAssertion to LookaheadAssertion since now it's also used in contexts where it cannot be followed by a quantifier (i.e. when UnicodeMode is enabled).

Note that [~UnicodeMode, +UnicodeSetsMode] can never happen, because of https://tc39.es/ecma262/#sec-parsepattern (and https://tc39.es/ecma262/#sec-parsepattern-annexb).

[nicolo-ribaudo]

It's also possible to reduce branching in this production, but I'm not sure it increases clarity as it mixes the +UnicodeMode and ~UnicodeMode branches:

        Term[UnicodeMode, UnicodeSetsMode, NamedCaptureGroups] ::
          [~UnicodeMode] LookaheadAssertion[~UnicodeMode, ~UnicodeSetsMode, ?NamedCaptureGroups] Quantifier
          Assertion[?UnicodeMode, ?UnicodeSetsMode, ?NamedCaptureGroups]
          [~UnicodeMode] ExtendedAtom[?NamedCaptureGroups] Quantifier?
          [+UnicodeMode] Atom[+UnicodeMode, ?UnicodeSetsMode, ?NamedCaptureGroups] Quantifier?

It could probably be further simplified by merging ExtendedAtom with Atom and moving the branching on UnicodeMode inside it, but the refactor becomes larger:

        Term[UnicodeMode, UnicodeSetsMode, NamedCaptureGroups] ::
          [~UnicodeMode] LookaheadAssertion[~UnicodeMode, ~UnicodeSetsMode, ?NamedCaptureGroups] Quantifier
          Assertion[?UnicodeMode, ?UnicodeSetsMode, ?NamedCaptureGroups]
          Atom[?UnicodeMode, ?UnicodeSetsMode, ?NamedCaptureGroups] Quantifier?

        Atom[UnicodeMode, UnicodeSetsMode, NamedCaptureGroups] ::
          `.`
          `\` AtomEscape[?UnicodeMode, ?NamedCaptureGroups]
          [~UnicodeMode] `\` [lookahead == `c`]
          CharacterClass[?UnicodeMode, ?UnicodeSetsMode]
          `(` GroupSpecifier[?UnicodeMode]? Disjunction[?UnicodeMode, ?UnicodeSetsMode, ?NamedCaptureGroups] `)`
          `(?:` Disjunction[?UnicodeMode, ?UnicodeSetsMode, ?NamedCaptureGroups] `)`
          [~UnicodeMode] InvalidBracedQuantifier
          [~UnicodeMode] ExtendedPatternCharacter
          [+UnicodeMode] PatternCharacter