Rename `environment-markers` to `resolution-markers` (#6240)
## Summary
I probably won't land https://github.com/astral-sh/uv/pull/6210 for the
release, but I do want to make one breaking change to prep for it:
renaming `environment-markers` to `resolution-markers` (or
`solution-markers`?) so that it's delineated from the user-defined
markers in that PR.
