Add automatic mode for -tl warning summary

[rainersigwald]

Continuation of #10681.

We should emit a summary if diagnostics were "long enough ago" in the scroll, as well as having the explicit opt-in/out.

[baronfel]

My proposal for a heuristic:

my ideal state would:

• know the number of vertical lines available the terminal (we should be able to get this data from a VT control code sequence)
• if that number is nonzero, estimate the number of lines we've written (NUM_PROJECTS * 1 + SUM(NUM_DIAGNOSTICS_PER_PROJECT)?
• if that number > TERMINAL_HEIGHT + SOME_PADDING_AMOUNT lines, then emit the summary

all of this would only occur if the summary had not been explicitly requested or disabled by a user.