refactor(utils): replace utils.open() with vim.open()

[troiganto]

Summary

This PR removes the function utils.open() and replaces it with vim.ui.open().

As far as I can tell, this helper was only used in export.lua. Hyperlinks used vim.ui.open() from the start.

The utils function does basically the same as the built-in, except:

1. doesn't return an awaitable object
2. doesn't search for executables as thoroughly.

Related Issues

Extracted from #878

Changes

• replaced utils.open() in export.lua with vim.ui.open()
• removed the function itself

Checklist

I confirm that I have:

• Followed the Conventional Commits specification (e.g., feat: add new feature, fix: correct bug, docs: update documentation).
• My PR title also follows the conventional commits specification.
• Updated relevant documentation, if necessary.
• Thoroughly tested my changes.
• Added tests (if applicable) and verified existing tests pass with make test.
• Checked for breaking changes and documented them, if any.

[kristijanhusak]

IIRC this util was for Neovim < 0.10 or some older version that didn't have vim.ui.open. It can be removed now since it's 0.10+.

Thanks for these cleanup PRs!