Add option to ignore case to checkFileExtension plugin

[bluej007]

Is your feature request related to a problem? Please describe.
I use the checkFileExtension plugin in flows to handle file extensions differently, and the current plugin sees mkv and MKV as different extensions though they are the same.

Describe the solution you'd like
Add a toggle or checkbox option to this plugin so it'll ignore case.

[CurlyFlow]

I dont think there is a case where case sensitivity would be useful, so we wouldnt need a checkbox for that?

[HaveAGitGat]

Updated here:
HaveAGitGat/Tdarr_Plugins#622

Didn't add a toggle as Tdarr is cross-platform and on Windows it's not possible to have both a lower and upper case extension for the same file.