Add timestamp view

# Description of the new feature/enhancement

iTerm2 has a very nifty functionality where you press a keyboard shortcut and it creates a sort of side-column (think of the line number column or the debugging column in an IDE) where it shows timestamps for all executed commands in its scroll back buffer, plus for all "significant events".

I think, but don't quote me on that, that it also shows timestamps for important events, for example if there's a time interval longer than X and suddenly there's a message, it shows the timestamp for that, too.

It's great for retroactively investigating something, especially if you have to use a shell which doesn't implement more advanced prompt features. Or for long running operations with sudden bursts of activity.

https://iterm2.com/documentation-one-page.html

> Toggle View > Show Timestamps to indicate the time each line was last modified. This is useful for telling how long operations took or when a message was printed.

# Proposed technical implementation details (optional)

You press a certain keyboard shortcut/select a menu entry.

An additional column is added to the left of the terminal, containing nicely formatted timestamps, down to the second (maybe millisecond, even?) that shows timestamps for all command executions plus significant events in the entire scrollback buffer, as marked by timing of printing to the screen (pause for longer than Y seconds - maybe make this configurable? - then the next message printed is significant and its timestamp is recorded and shown).

No source file or test is named. Start by tracing the terminal's scrollback rendering and keyboard/menu entry points, then compare the requested behavior with the linked iTerm2 description. Done means a toggleable timestamp column records command and significant-event times across the scrollback buffer.

Written by the indexing model from the issue text.