This works by stripping the optional chaining characters from the completion part variable, allowing the developer tools' parser to proceed as if it were a regular, non-optional expression.
Tests were partially based on: https://bugzilla.mozilla.org/show_bug.cgi?id=1594009
Tests for features that do not apply to our version of developer tools (e.g. autocomplete for integer literals, ignoring spaces between property accessors, etc) were excluded.
