Major mode for CUE sheet cue files.

The official standard supports only a few keywords but in principle many more
are in use through REM. This mode treats both the proper keywords and the
ones commonly in use in the wild through REM as regular keywords.

This mode was previously called `cue-mode' but that clashes with the mode on
MELPA used for the CUE data validation language unrelated to this.

Additional documentation of the format:
https://www.gnu.org/software/ccd2cue/manual/html_node/CUE-sheet-format.html