Improve lexing of unclosed escaped strings

[rubinovitz]

Fixes #2109.

This avoids running the general string regular expression when the lexer is already positioned at a quoted string inside a block, variable, or line statement. A small scanner finds the closing quote while respecting escaped quotes, or raises the same syntax error path immediately when the string is unterminated.

This keeps the existing regex for compatibility, but skips its backtracking-heavy failure case for unclosed strings with many escape sequences.

Validation:

• python -m pytest tests/test_lexnparse.py -q -> 151 passed
• python -m pytest -q -> 912 passed
• issue-style local reproducer: ~0.0899s -> ~0.0191s
• Tenfold Docker eval for the Jinja poor performance when parsing unclosed string with many escape characters #2109 bounty: 0.007996s