Improved source to pdf search in pdf-tools

[Rahguzar]

Hi everyone,

I recently implemented improved source to pdf (aka forward) search in
pdf-tools. It can be found at
https://github.com/aikrahguzar/pdf-tools/tree/child-frame-preview

It should have word level accuracy in text and should highlight
something close to the current word in math. In rare cases it falls back
to the behavior currently present in upstream pdf-tools.

Due to limitations of synctex support in epdfinfo this requires the
synctex executable to be present. See
https://github.com/vedang/pdf-tools/issues/244#issuecomment-2481081215

There are probably rough edges and bug. So if you want this
functionality please try it out and report any problems.

P.S. This branch also has improved backward search compared to upstream
pdf-tools. I don't fill lines and use visual-line-mode for wrapping.
Upstream pdf-tools doesn't play well with this but with my changes long
lines don't cause problems with sync.

Rahguzar