[Feature] "Show PDF" button

[dvdvgt]

This PR is in response to #69.

Discussion

I am not really happy how the file extension/type is currently manually changed from .typ to .pdf. I'll happily implement something more robust if somebody has a recommendation.

[nvarner]

At present, I think the filename conversion is okay. Doing better depends on finding a good way to decide which files get compiled into PDFs and what those PDFs should be called, which hasn't happened yet.

[nvarner]

@dvdvgt can you fix the failing lint? This will be ready to merge after that.

[dvdvgt]

Should be good now. Feel free to squash all commits into one.

[nvarner]

Oops, that latest failure might have been my fault; I needed an extra feature in a dependency, and I force pushed while preparing a PR to upstream the changes. My changes were upstreamed but not yet released, so it's okay to keep the elsa Git dependency. To fix the changed Git commit hash, can you try cargo update elsa? I think that will resolve it.

[nvarner]

Thank you!