Add Certora syntax highlighting (#5861)

Co-authored-by: Hadrien Croubois <hadrien.croubois@gmail.com>

.gitattributes

@@ -0,0 +1,3 @@
+certora/specs/*.spec linguist-language=Solidity
+certora/specs/*.conf linguist-detectable
+certora/specs/*.conf linguist-language=JSON5