feat: add title_pos ui option

[thomaswhyyou]

Adds a title_pos ui option to control the popup title alignment.

I find it easier to understand the file paths I'm tracing when they are aligned, but I did not find an option to change this.

Note: I used Claude to produce this change. I'm not experienced in nvim plugin development, but looking over the diff it looks to be a pretty straightforward change. I tested it locally and it works as intended.

Thanks for making this plugin, it's really good. Please feel free to close this PR if this isn't something you are willing to accept.

[WilliamHsieh]

LGTM, thanks for the contribution.

Note: doc/*.txt is auto-generated from the annotations in config.lua (CI runs make vimdoc), so you don't need to edit it by hand, but no harm done here since it matches.