Improve mosh predictive echo + fish autosuggestion interactions

This issue tracks the experience of using the fish shell over mosh. #1363 is the fish-shell issue.

The problem is the interaction with autosuggestions and predictive local echo. Autosuggestions are proposed completions to what the user types, that appear after the cursor in a muted color. A picture should make it clear. Crucially text at the insertion point will always overwrite the autosuggestion. The problem is that mosh's predictive echo instead inserts the text before it.

Example showing an autosuggestion:

as the user keeps typing, the autosuggestion is predictively pushed instead of overwritten, until the client receives the new frame:

Overall there tends to be a lot of distracting churn/noise to the right of the cursor on high-latency connections.

Predictive echo is really nice and we would like to make it work well with fish. The ideal experience would be for mosh to "predictively erase" the autosuggestion whenever it inserts a predictive character. I don't know how feasible that is, but on the fish side we're happy to do things that will help mosh, like emitting escape sequences.

Reproduce the interaction by using fish over mosh on a high-latency connection, then read fish-shell issue #1363 alongside the mosh predictive-echo behavior described here. Done means predictive echo no longer causes distracting autosuggestion churn while typing, with any required coordination between mosh and fish identified.