Replace ledger-get-minibuffer-prompt with built-in format-prompt

[bcc32]

This introduces a dependency on the compat package. Not sure if the other
maintainers think that might be controversial, but doing so would allow many
other things to take advantage of more modern APIs, and faster, while retaining
compatibility with older Emacs versions.

[bcc32]

(requesting some review due to the introduction of the compat dependency)

[purcell]

package-lint doesn't need the dependent packages to be installed, it just needs to know they are installable.

[purcell]

I'm a bit on the fence about this. On the one hand it would likely be fine to raise the minimum Emacs version required by ledger-mode to 27.x, or perhaps even 28. Requiring extra packages is slightly more drastic, and while I'm happy compat exists, I don't think it always provides perfect compatibility. I'm not hugely opposed though — depends a bit on what improvements it could yield here.