vim: Fix indentation at global scope after non-semantic ([{/)]}

[lilyball]

If an unbalanced [ exists in a string or comment, this should not be
considered when calculating the indent at the top level.

Similarly, when testing for ({/}) to see if we're at the top level to
begin with, strings and comments should be skipped.

[lilyball]

cc @chris-morgan

[lilyball]

r? @thestinger

I had to tweak it, turns out the other searchpair() call can benefit from this same thing. I skipped it before because I wasn't sure if it was necessary, but I found a reproduction case inside of str.rs,