Add support for bpftrace-mode

[jgkamat]

I created a new mode for bpftrace, which is based on c-mode, and this patch adds support for indentation via c-basic-offset.

[xuhdev]

@10sr I see no problem with this PR. What do you think?

[10sr]

Thanks! 👍

FYI, for this case it should work without this PR because editorconfig can find parent major-modes properly, but it is OK (or maybe better) to define explicitly like this.

[jgkamat]

10sr writes:

FYI, for this case it should work without this PR because editorconfig can find parent major-modes properly

oh whoops, it turned out I was editing some files above my root level editorconfig file, and I thought this was needed to add support (when the files were just in the wrong place). Feel free to revert this if you want to tidy the list at any point, although it shouldn't make any difference.