Fix key parsing for explicitly major keys.

`Vex.Flow.Music#getKeyParts()` is broken for major keys that explicitly use the suffix "M", rather than relying on major as the default when no key type is specified. For example, "CM" is interpreted as C minor, rather than C major as expected.

This is because the entire key string is lowercased before parsing, including the key type suffix. With this change, only the note-name portion of the key is case-insensitive, to properly distinguish between key types "m" and "M".