Improve handling of multi-line quoted values in the fallback INI parser

[sgiehl]

Summary

The fallback INI parser in IniReader (used when PHP's native parse_ini_string() is not available) parsed the input strictly line by line. It therefore did not recognise quoted values that span multiple lines: the lines after the opening quote were treated as separate sections or keys, while the native parser reads them as part of the value.

This aligns the fallback implementation with the native parser and makes values round-trip reliably through IniWriter/IniReader.

Changes

IniReader

• The fallback implementation reads multi-line quoted values — both " and ' — up to the matching closing quote, so both implementations return the same result. The value is scanned once while it is read, not from its beginning again for every line.
• A quote at the very end of a line ends a value that would otherwise be unterminated, so values written by an earlier version of IniWriter are still read as before.
• The value is taken from the unmodified line, so tabs and inner whitespace are kept, and \r\n counts as one line break so it is preserved inside a value.
• Anything after the closing quote of a value, such as an inline comment, is ignored.
• Unterminated quoted values are rejected, consistent with the native parser.
• readComments() skips multi-line values as well, so it no longer reports sections and keys that readString() does not return.

IniWriter

• Backslashes are escaped in addition to quotes, so a value ending in a backslash cannot leave the closing quote ambiguous. IniReader reverses this when reading.
• All quotes directly before a line break are removed, so none of them can end up as the last character of a line.
• Option names are encoded like array keys and section names, keeping characters that are valid in a name such as . or :.

Tests

Added tests that run against both the native and the fallback implementation, covering multi-line double- and single-quoted values, values containing newlines, tabs, \r\n, backslashes (e.g. Windows paths) and INI-significant characters, option names, content after a section name or a closing quote, and unterminated quotes.

vendor/bin/phpunit: 140 tests, 226 assertions, all passing (verified on PHP 8.3; CI covers 7.2-8.4).

Reading Matomo's bundled global.ini.php and an instance config.ini.php produces byte-identical results before and after this change, on both implementations, and readComments() returns the same descriptions as before.