vtparse: add method to determine ground state

Add `VTParser::is_ground`, which is true if the VTParser is in the
"ground" state, i.e. it has no stored state that will affect the
interpretation of future input characters.

vtparse/src/lib.rs

@@ -432,6 +432,12 @@ impl VTParser {
         }
     }
 
+    /// Returns if the state machine is in the ground state,
+    /// i.e. there is no pending state held by the state machine.
+    pub fn is_ground(&self) -> bool {
+        self.state == State::Ground
+    }
+
     fn as_integer_params(&self) -> [i64; MAX_PARAMS] {
         let mut res = [0i64; MAX_PARAMS];
         let mut i = 0;