Use the syntax table to detect if point is in a comment.

This fixes #8538, where `julia-comment-open` was getting confused by
strings inside comments.

It also generalises `julia-in-comment` to handle multi-line comments.
Respect the active region when indenting.

This fixes #8634.
Merge pull request #8633 from Wilfred/further_indentation_improvements

Indentation fixes for julia-mode