Consider allowing `abs_sub`

[ada4a]

In L11L03 (and I'm pretty sure in some previous level(s) as well), we essentially derive abs_sub:

RealAnalysisGame/Game/Levels/L11Levels/L03_IsBddOfCauchy.lean

Lines 67 to 68 in 5584aa2

	have f2 : |a m| = |(a m - a N) + a N| := by ring_nf
	have f3 : |(a m - a N) + a N| ≤ |a m - a N| + |a N| := by apply abs_add

It would be nice to have access to that theorem -- one could imagine its derivation being a (rather simple) problemset exercise