[Style] Enforce a .. b opSlice style

[wilzbach]

Yet another check & automatic conversion.

@andralex I mainly did this because you keep nagging about this on PR reviews and there's really no point in wasting time during a review with such easily to check nits.
(for the record I prefer the a..b)

There were a few false positives with this very naive approach, hence I excluded the std.string and std.uni for the start.
Moreover, I hope you are aware of the following:

import std.conv, std.uni;
assert(unicode.ASCII.to!string == "[0..128)");

[andralex]

@wilzbach I'm getting increasingly worried about false positives due to the regex-based enforcement. Are you considering rolling these into dscanner?

[wilzbach]

@wilzbach I'm getting increasingly worried about false positives due to the regex-based enforcement.

I am worried about that as well (this is only intended to be an intermediate solution).
In the worst case we can exclude specific files from a specific regex as done here.

Are you considering rolling these into dscanner?

Of course, but the PR is pending since quite a while: dlang-community/D-Scanner#348