Fix opening file with colon (zed-industries#17281)

Closes zed-industries#14100

Release Notes:

- Fixed unable to open file with a colon from Zed CLI

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I didn't make change to tests for the first two commits. I changed them
to easily find offending test cases. Behavior changes are in last commit
message.

In the last commit, I changed how `PathWithPosition` should intreprete
file paths. If my assumptions are off, please advise so that I can make
another approach.

I also believe further constraints would be better for
`PathWithPosition`'s intention. But people can make future improvements
to `PathWithPosition`.