feat: Reject with keyword usage

RVM does not plan to support the `with` keyword which is mainly used
for testing.

- introduce CompilerError::WithKeywordUnsupported and fail query compilation
  when any literal carries with_mods
- skip OPA test cases that hit the error

The "withkeyword" folder is retained in the TODO list to indicate its
lack of support.

Signed-off-by: Anand Krishnamoorthi <anakrish@microsoft.com>

src/languages/rego/compiler/error.rs

@@ -15,6 +15,9 @@ pub enum CompilerError {
     #[error("Unknown builtin function: {name}")]
     UnknownBuiltinFunction { name: String },
 
+    #[error("the `with` keyword is not supported by the compiler yet")]
+    WithKeywordUnsupported,
+
     #[error("internal: missing context for yield")]
     MissingYieldContext,