Problem: I'm trying to write a command that takes the selected text and appends it the end of a file of my choice.
There also needs to be a tab completion (or autocompletion) element to the command, in the sense that the command should be able to autocomplete to files that are nested somewhere within the directory
Example: Suppose there is a file
/tab/completion/directory/nested/file.md, and I select text in my buffer and run
M-x append-to-file, then I should be able to
M-x append-to-file file.md<TAB> for it to autocomplete to
M-x /tab/completion/directory/nested/file.md. After running this command, the selected text should be sent to end of the file.