Improve handling of pressing ``Esc`` during message compose

[srdeotarse]

What does this PR do, and why?

Adds a confirmation popup to save composed message as a draft before exiting the message compose to avoid accidental exit while typing a message and loosing it.

External discussion & connections

• [x] Discussed in #zulip-terminal in Improve handling Esc during msg compose #T1342 #T1362 #T1442
• [x] Fully fixes #1342
• [ ] Partially fixes issue #
• [ ] Builds upon previous unmerged work in PR #
• [ ] Is a follow-up to work in PR #
• [ ] Requires merge of PR #
• [ ] Merge will enable work on #

How did you test this?

• [x] Manually - Behavioral changes
• [x] Manually - Visual changes
• [ ] Adapting existing automated tests
• [ ] Adding automated tests for new behavior (or missing tests)
• [ ] Existing automated tests should already cover this (only a refactor of tested code)

Self-review checklist for each commit

• [x] It is a minimal coherent idea
• [x] It has a commit summary following the documented style (title & body)
• [x] It has a commit summary describing the motivation and reasoning for the change
• [x] It individually passes linting and tests
• [ ] It contains test additions for any new behavior
• [x] It flows clearly from a previous branch commit, and/or prepares for the next commit

Visual changes

[zulipbot]

Hello @zulip/server-hotkeys members, this pull request was labeled with the "area: hotkeys" label, so you may want to check it out!

[neiljp]

@srdeotarse Thanks for getting work started on this :+1: Closing this PR now, since the active #1442 was merged.