feat: add Typst PDF export command

[davidsanchez222]

Hi, I have been using this project to edit my resume in Typst/Neovim. It has completely removed my need to use Word to make/edit my resume which is a pain. The Typst/Neovim workflow is much more streamlined and a joy to use. Thanks for making this project its awesome! :)

One feature that I think is missing is being able to export the .typ file in the buffer to a PDF. Sure, one can do this with the typst CLI with:

typst compile filename.typ

But having a command in Neovim that I can bind to a keystroke makes my workflow faster.

The command I added is called
TypstPreviewExport

and it saves the .typ file to a PDF with the same name unless an output is specified in which case calling it would be TypstPreviewExport specifiedName.pdf

I rewrote the logic for get_path in commands.lua to pull it out as a shared helper so both preview and export use the same path/unsaved‑buffer checks while allowing action‑specific error messages. It's the same behavior as before, just moved to a shared helper so export_pdf can use it in a clean manner.

Thanks!