feat: solve day 7

We parse the grid and then step through a simulation of the tachyon
beam's spread and count the number of splitters that the beam actually
hit.

aoc25.cabal

@@ -75,3 +75,10 @@ executable aoc25
 
     -- Base language which the package is written in.
     default-language: GHC2021
+
+executable day7
+    import:           warnings
+    main-is:          Day7.hs
+    build-depends:    base ^>=4.20.2,
+                      containers >= 0.6.7
+    hs-source-dirs:   app