shell integration: Elvish doesn't mark OSC133 input regions

[mitchellh]

See below:

The full prompt line should not be orange. It should mark input as blue and the right prompt should probably be marked k=r.

[jparise]

Making this work reliably might be challenging given that Elvish renders its prompts on a background thread and also filters most escape sequences from the output.

https://elv.sh/ref/edit.html#prompts

I've made some (local) improvements, but it might not be possible to get a full implementation working with only the existing hooks.

Alternatively, there's an upstream issuing to implement OSC 133 directly in the shell (elves/elvish#1808), so that's probably the best long-term solution.

[jparise]

elves/elvish#1917 implements OSC 133 directly in Elvish.