Add support for T/t and P/p in fretboard fingerings (Fixes #23755)

[pyonpyoco]

Resolves: #23755

Description:
This PR adds support for entering T/t (thumb) and P/p (pulgar) as fingering values for fretboard diagrams.

Changes:

• Updated DOM fingering parsing and internal values.
• Mapped T/t → 6, P/p → 7 internally to keep the existing data structure.
• Updated Inspector UI to allow entering T and P.
• Updated reading/writing logic to support these values.

Backward compatibility:
Opening scores using this feature in older versions (4.6.3) of MuseScore currently displays the digits 6 and 7.
Please advise if backward compatibility handling or a conversion layer is preferred.

UI example

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