[fix] Error indentation offset got with non-number type variable

[hikycool]

Some indent-offset variable is not a number type for default value. Example: In c++-ts-mode, "c-basic-offset" is default type of "symbol" and default value is "set-from-style". And in the function of "copilot--infer-indentation-offset" will return value of "set-from-style".

Change: Add a new judgment before returning indent-offset