Organize test-cases into folders, extract compatibility test into proper test, support adding test-case specific tests

betterproto/tests/inputs/keywords/keywords.json

@@ -0,0 +1,5 @@
+{
+  "for": 1,
+  "with": 2,
+  "as": 3
+}

betterproto/tests/inputs/keywords/keywords.proto

@@ -0,0 +1,7 @@
+syntax = "proto3";
+
+message Test {
+  int32 for = 1;
+  int32 with = 2;
+  int32 as = 3;
+}