feat: add certora CI integration (#10)

This adds a new command to package.json `verify` which can be run via
`pnpm verify`.

The command runs the certora CLI with a config file which has to be
adjusted for every individual project.

The commit also adds a dedicated task to our github actions, which
ensures, verification is done in every PR as well.

certora/certora.conf

@@ -0,0 +1,8 @@
+{
+  "files": ["src/Foo.sol"],
+  "msg": "Verifying Foo.sol",
+  "rule_sanity": "basic",
+  "verify": "Foo:certora/specs/Foo.spec",
+  "wait_for_results": "all",
+}
+

certora/specs/Foo.spec

@@ -0,0 +1,9 @@
+methods {
+  function id(uint256) external returns (uint256) envfree;
+}
+
+rule checkIdOutputIsAlwaysEqualToInput {
+  uint256 input;
+
+  assert id(input) == input;
+}