Match files by a prefix

[OJarrisonn]

It would be a cool addition to match files not only by their extension, but also by a prefix.

For instance, when I'm running rsp with fzf to see my history, I often see history entries like $env.SOME_ENV_VAR (i use nushell) so i'd like to add a previewer so i could see the value of this given environment variable

For instance the config could look something like:

[filetype.envvar]
prefixes = ["$"]