Set adaptive-fill-regexp and fix comment-start-skip.

[mychris]

Configures the adaptive-fill-regexp for zig-mode, such that other commands like fill-region can automatically determine a fill-prefix for comments.

Also, adds doc comment style comments to comment-start-skip, so these commands also work in doc comments.

Closes #86

[precompute]

Ran into this issue and this fixes it. fill region works as it should after this patch.

To test, eval (fill-context-prefix (point) (point)) on a comment via M-:.