textarea: support dynamic prompts

[knz]

The secondary prompt is displayed on every line after the first.
See for example:

Used in https://github.com/knz/bubbline

cc @maaslalani

[knz]

ping @maaslalani

[maaslalani]

Hey @knz,

This change seems very specific for line editing, the textarea bubble is meant to emulate the HTML textarea so that people are familiar with it and it is easy to understand / use.

https://developer.mozilla.org/en-US/docs/Web/HTML/Element/textarea

I can totally see why this is useful for a line editing application.

One more generic implementation that comes to mind (not saying this is the correct approach) is to perhaps allow a PromptFunc to be defined that passes the line number and allows the prompt to be defined as a function of the line number (func (lineNumber int) string) that way it is at least a bit more generic i.e. people can implement alternating prompts for even or odd lines, or hide the prompt past line 5, or make the first and last prompts a special characters, and this next prompt logic could be implemented.

[knz]

PromptFunc works for me!

[knz]

i'll make the change

[maaslalani]

Thanks @knz, much appreciated!

[knz]

Hi @maaslalani I have changed the logic to use a dynamic prompt as you suggested!

[maaslalani]

This looks absolutely fantastic @knz, thank you so much!

Another use case I just thought of for having the PromptFunc take a line number is to show diagnostics / error warnings for certain line numbers that have parse errors on them. So this change will be extremely useful for that!