Replace the nested-quantifier string pattern /"(?:[^"\\]*|\\.)*"/
with /"(?:[^"\\]|\\[\s\S])*"/ in the Lisp/Elisp grammar.
The original pattern could catastrophically backtrack (O(2^n)) when a
string contained a backslash-newline line continuation, because JS dot
does not match newlines, leaving both alternatives unable to advance.
The fixed pattern eliminates nested quantifiers and uses [\s\S] so
line continuations are consumed correctly.
Adds a regression test with a 5s timeout guard and populates the
previously-empty fixtures/commonlisp/*.el fixture files with realistic
Elisp code that includes the backslash-newline pattern.
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