getline: Support prefill line buffer

[tw4452852]

This is especially useful when line buffer could be prefilled by an external program and then be edited later.

BTW, this also paves the way for testing of getline, although we need to work around rawterm stuff first.

[sogaiu]

I'm not sure that changing this bit:

  (fn getline-fn
    [&opt prompt buff _]

to:

  (fn getline-fn
    [&opt prompt buff prefill]

is a good idea.

IIUC, (fn getline-fn ...) is what make-getline returns. Not sure, but I think the _ argument for (fn getline-fn ...) is space for an "environment".

An example of this can be seen here in the call to gl:

spork/spork/netrepl.janet

Lines 352 to 356 in 4224d56

	(def gl (make-getline nil get-completions get-docs))
	(forever
	(def p (recv))
	(if-not p (break))
	(def line (gl p @"" root-env))

i.e.

(gl p @"" root-env)

...though may be I'm missing something 😅

[tw4452852]

Hmm, You're right. I originally thought this parameter is just reserved for future use. Maybe we should add another parameter?

[sogaiu]

Not sure 😅

Random thought...use a dynamic variable? Don't know if that's a good idea.

May be @bakpakin would have better thoughts about this.