Fix previews (video, pdf) and support uppercase file extensions

[jnk22]

Hi, I have just tried to setup this plugin in NeoVim.

First of all, thank you for keeping this plugin alive @Conni2461 and everyone else involved!

It seems there are a few problems with file previews in its current version (possibly partly related to the breaking change #35):

1. Video preview does not work when viu is not installed
2. PDF preview does not work due to preview script not finding the generated preview files
3. Files with non-lowercase extensions are not supported

This PR is supposed to fix those things:

1. As viu does not seem to be used for any previews (only chafa is), I have replaced the check for its existence with a check for ffmpegthumbailer which seems to be originally intended for that line.
2. pdftoppm already appends the file extension .png when using the command option -png (This is at least true for my installed version, 23.03.0 - this might need double checking for earlier versions). Therefore I have removed the additional .png for the output file.
3. When previewing image files with file extensions like .JPG or .MP4, I get the error message: unknown file ..image_path.JPG. I have added a line to make the extracted extension lowercase to match the following case patterns.

@HendrikPetertje Could you verify if these changes still work for you?

[Chiarandini]

got the same problem, this fixes it -- oh shoot, I just meant to comment

[sudo-burger]

Works fine here (MacOS + brew (newer bash than what comes with MacOS) + neovim v0.9.5).
The substitution "${extension,,}" doesn't work with the stock MacOS bash (v3.2.57(1) at the moment).