Does this command (imperative) sentence sound natural?
Use emacs as the default text editor.
Also, is the article "the" necessary?
Emacs is the name of a text editor.
Zoltán KirályDoes this command (imperative) sentence sound natural?
Perfectly.
Zoltán KirályAlso, is the article "the" necessary?
Yes.
Comments
"Emacs" should be capitalized. The sentence is OK if it is directed at the programmer who will be setting the default. If it is directed at a user, not so much.
It is not strictly necessary grammatically, but it is much better to use it in this case. Clarity is everything in technical writing.