feat(sterni/exercises/lib): add Diagonals BQN function

Retrieves all diagonals of a two dimensional array (which have differing
lengths). Useful to solve e.g. Project Euler problem 11
(<https://projecteuler.net/problem=11>).

Change-Id: I853bc6eaaf869326d84d8e789fee9995f5a332d8
Reviewed-on: https://cl.tvl.fyi/c/depot/+/12897
Autosubmit: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

users/sterni/exercises/aoc/lib.bqn

@@ -16,3 +16,6 @@ _fix ⇐ {𝕩 𝕊∘⊢⍟≢ 𝔽 𝕩}
 ImportBqnLibs ⇐ {•Import 𝕩∾˜"/"∾˜¯1↓1⊑•SH "printenv"‿"BQNLIBS"}
 
 Xor ⇐ (¬⊸∧∨∧⟜¬)
+
+# To get all Diagonals (⊢∾○Diagonals⌽)
+Diagonals ⇐ {(0‿0⊸⍉ ↓⟜𝕩)¨ (↕≠𝕩)}