Make relative effectful

[m-rinaldi]

Description of the change

relative is typed as a pure function, but Node's path.relative resolves both of its arguments against process.cwd() before computing the relative walk. Whenever either argument is a relative path, the result therefore depends on the working directory at the time of the call:

cwd=/           relative "a" "../b"  ->  "../b"
cwd=/usr/local  relative "a" "../b"  ->  "../../b"

This is the same reasoning that moved resolve into Effect in #14 (see also #4 and the discussion in #20). path.relative calls path.resolve internally and inherits the dependence, but its signature was never updated.

This PR changes relative to FilePath -> FilePath -> Effect FilePath, with the FFI written the same way as resolve. The doc comment now mentions the working-directory dependence, and the existing test is adjusted to bind the result.

This is a breaking change and will need a major version bump.

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Checklist:

• Added the change to the changelog's "Unreleased" section with a reference to this PR (e.g. "- Made a change (#0000)")
• Linked any existing issues or proposals that this pull request should close
• Updated or added relevant documentation
• Added a test for the contribution (if applicable): the existing relative test was updated for the new type; no new test was added, matching resolve is impure #14.

[garyb]

Absolutely, good catch! Surprised it's taken anyone this long to notice 😄

[m-rinaldi]

Thanks! Felt like a déjà vu of #14 🔁