Support negative line numbers for goto

[barkure]

Summary

Implements the request from #684.

• Support negative line numbers for file goto targets, e.g. edit README.md:-1 opens at the last line and edit README.md:-5 opens at the fifth line from the end.
• Apply the same negative line behavior to the interactive Go To dialog, so CLI and in-editor navigation stay consistent.
• Keep columns positive-only; values like README.md:10:-5 are not treated as goto targets.

Test plan

• Built successfully with:

cargo build -p edit --release

• Ran parser coverage with:

cargo test -p edit documents::tests::test_parse_last_numbers

• Manually tested:
  target/release/edit README.md:-1
  target/release/edit README.md:-5