fix #321924 allow sub and sup tags in instrument names

[wizofaus]

Resolves: https://musescore.org/en/node/321924

Use of sub- and superscript in text including instrument names is very common and already supported by the rendering engine.

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[wizofaus]

Surprisingly this function is currently ONLY used for validating instrument/staff names. Note that there's a similar function TextBase::escapeTag that isn't used anywhere.
As far as I can tell there are no unit tests that call this, though I'd think there should be.