ixml parser: attribute values may be empty or contain any character

[kenzhemax]

The attribute regex whitelists the characters a value may contain, so parsing stops with ASSERTION_FAILED on values Excel writes into xlsx files:

• <x15ac:absPath url="C:\Users\me\"/> in workbook.xml (backslash)
• <a:ea typeface=""/> in theme1.xml (empty value)
• a sheet name with non-English letters such as é, ş or қ (JS \w is ASCII only)

Fix: a value is anything up to the closing quote, [^"]*.

Testcase included, fails on current main with ASSERTION_FAILED in if_ixml_parser~parse. Full test suite passes.

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