Fix disallowed_script_idents FP on identifiers with _

[profetia]

Closes #15116

The cause for this issue is that _ belongs to the Common catagory in unicode instead of Latin like other ASCII alphabets. Since ASCII characters are always allowed, I just added an extra is_ascii() check to ensure this.

changelog: [disallowed_script_idents] fix FP on identifiers with _

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