Allow a variable number of sections in bossbars, rather than preset choices

Currently, when typing /bossbar set <id> style, you are presented with preset options for the number of sections. There are 4 notched options, and progress, which sets the number of sections to 1.

It crosses my mind why it was decided that only the preset choices would be allowed. In fact, I see no reason why we can't let command users enter their own custom number for the number of sections instead.

How about, instead of bossbar set <id> style (notched_6|notched_10|notched_12|notched_20|progress), we have instead the following command:

/bossbar set <id> sections <sectionCount>

and allow users to set their own preferred number of sections instead.