std.Uri: fix implementation of resolve with trailing slashes

[truemedian]

This should solve #18291.

This required adapting resolvePosix for Uri into its own function. This also has the benefit of never returning relative paths, which used to be possible.

[yzongyue]

I'm more inclined to re-implement resolvePosix in uri.zig to avoid process path multi times (don't mind me)

[truemedian]

removeDotSegments is the reimplementation of resolvePosix, but this time following the rules laid out in the RFC.