Fix absolute path detection for build on save diagnostics

[jayrod246]

This patch detects drive letters.

I was trying to trigger some diagnostics using std.fmt. For awhile I couldn't tell if anything was working at all. But after a debug session, I figured out that the ':' following a drive letter was causing the line iterator to bail.

After this change I was able to see this:

[Techatrix]

This check is a bit hacky but shouldn't cause any problems. I anticipate the build on save diagnostic system to be reworked anyway.

[jayrod246]

Yeah I anticipated a rework when I was playing around with this.

I experimented with writing an actual parser for Zig compiler output, that tries to notice note:s for example that may appear on lines that follow a compile error. But this worked to get most diagnostics to show up for me.

[Techatrix]

The : can be very annoying. Especially because it can be part of a valid filename like /somewhere/:.zig:2:5: error: ...
I would say that the best way forward is to wait for integration with Zig's compiler protocol which can send errors as std.zig.ErrorBundle. They don't require any parser which makes them far easier to deal with.