Add web app hotkeys for message formatting during compose

[neiljp]

Description of feature

This could include various hotkeys, which may or not be appropriate to the terminal environment.

These may conflict with existing keys, but we should collect them when we know them, and if not added, then at least note somewhere that they are not present (or why) - or have a different hotkey, or workaround.

Known at this time: (list likely incomplete)

• pasting in text (see related possible bugs re eg. tab/enter?)
• ctrl + shift + l (ctrl+L): while text selected -> wraps link syntax ready to paste text
• cmd/ctrl + shift + c: shortcut for inserting code span (zulip/zulip#29646, zulip/zulip#30533)

[zulipbot]

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