Tests for basefee as identifier in inline assembly

Also added basefee to `EVMVersion::hasOpcode(...)`

test/libsolidity/syntaxTests/inlineAssembly/basefee_reserved_london.sol

@@ -0,0 +1,12 @@
+contract C {
+    function f() public view returns (uint ret) {
+        assembly {
+            let basefee := sload(0)
+            ret := basefee
+        }
+    }
+}
+// ====
+// EVMVersion: =london
+// ----
+// ParserError 5568: (98-105): Cannot use builtin function name "basefee" as identifier name.