feat(ui): add `open_split_vertical` option for splits opened

[dwtong]

when opening splits from a floating window with the "Open in split" option, this will set the split to open vertically.

configure with:

vim.g.rustaceanvim = {
    tools = {
        float_win_config = {
            open_split = 'vertical',
        },
    },
}

[github-actions[bot]]

Review Checklist

Does this PR follow the Contribution Guidelines? Following is a partial checklist:

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• For example, fix(lsp): some lsp-related bugfix

• [ ] Pull request title has the appropriate conventional commit prefix.

If applicable:

• [ ] Tested
  • [ ] Tests have been added.
  • [ ] Tested manually (Steps to reproduce in PR description).
• [ ] Updated documentation.
• [ ] Updated CHANGELOG.md

[dwtong]

i have made the requested changes - if you're happy with them then it's all ready to go :)