Check logic in IsVariableName

[bgrainger]

MySqlParser.IsVariableName attempts to detect a legal variable name. According to User-Defined Variables:

the variable name var_name consists of alphanumeric characters, ., _, and $.

However, Schema Object Names allows "Extended: U+0080 .. U+FFFF" in unquoted identifiers.

According to #193, variable names such as @ö are allowed by MySQL Server but not detected by IsVariableName, which may indicate a bug in the implementation.

[bgrainger]

Fixed in 0.48.0.