Fix Alt modifier on named keys

[happykawayigt]

Fixes #6663

Summary

• preserve the Alt prefix for named keys parsed from ESC-prefixed terminal input
• keep uppercase shift normalization scoped to single-character keys
• add regression coverage for alt+enter and alt+question_mark

Testing

• .venv\Scripts\python.exe -m pytest -q tests\test_xterm_parser.py (68 passed, 1 xfailed)
• .venv\Scripts\python.exe -m black --check src\textual\_xterm_parser.py tests\test_xterm_parser.py
• direct parser reproduction now yields ["alt+enter"]
• git diff --check