Day 19 solution

Cribbed heavily from https://www.reddit.com/r/adventofcode/comments/rjpf7f/comment/hp5k554/?utm_source=share&utm_medium=web2x&context=3 because this problem breaks my brain in so many ways. I understand the approach, but I got twisted up in my own head trying to write down the solution, so instead I started from a working copy and optimized it (a little bit) to make sure I understood what it was doing.

advent-of-code-2021.csproj

@@ -72,6 +72,9 @@
 	  <None Update="inputs\18.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>
+	  <None Update="inputs\19.txt">
+	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
+	  </None>
 	  <None Update="inputs\20.txt">
 	    <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
 	  </None>