Generic command to add shell prelude

[eraserhd]

The idea: a command like so:

define-shell-prelude kakquote %{
  kakquote() {
    : # implementation
  }
}

When the word specified in the first arg (kakquote) is encountered in a shell expansion, the text of the second arg is prepended to the shell expansion. The text could be source /path/to/file.sh.

It could be adding an element to $PATH, allowing binaries to be shipped with plugins without needing an option to compute their full path. (Although it might be better to have a facility just to set $PATH.)

This could be provided in a module which is required by other modules.

Transitive dependencies (other prelude blobs and variables used by them) can be precomputed and updated when a define-shell-prelude command is read to improve processing time, if needed.

• Add a kakquote function to the prelude of shell blocks #3340 suggests a kakqoute function added to the prelude of every function.
• Implement a portable readlink function #1224 suggests a custom readlink function.
• The join() function appears 12 times in rc/
• Tmux version detection code used to be copied in two places.
• @mawww's fd 3 idea might warrant some best practices for communicating with the fd that could be prelude.

The main idea here is to avoid including unused prelude (for example, prepending kakquote to every shell invocation, even when it isn't used), while still allowing the reuse of shell functions, including those provided by plugins.

[lenormf]

This mechanism looks very brittle, and could potentially backstab users who are not aware some code is being injected into their scripts.

Making users explicitly source a file shipped with Kakoune seems like a safer solution, especially if the hypothetical implementation of the commands being injected is going to be just a call to source.

Or ship full scripts and let the user fix their $PATH, c.f. #1430.