feat: add "extension" linemode (m e) to show extensions of files

[mikavilpas]

Solves #903

[sxyazi]

Thanks for the PR, but I don't like solving it in this way:

• When the file name is fully displayed, two extensions appear, which is a waste of space
• Still causing unexpected truncation when the filename is long enough (possibly not just the extension, but also the filename)

I want to implement it like "long-file-name.mp4" to be "long-fil~.mp4"

[mikavilpas]

I see. Do you think this should be the new default behavior, or toggled on with the linemode setting?

[sxyazi]

Yeah it should be the default behavior