[0.3.0-draft] add fields.get-and-delete

This saves the user from having to use `get` followed by `delete` if they want
access to the values they're removing.

Signed-off-by: Joel Dice <[email protected]>

wit/types.wit

@@ -195,6 +195,17 @@ interface types {
     /// syntactically invalid.
     delete: func(name: field-key) -> result<_, header-error>;
 
+    /// Delete all values for a key. Does nothing if no values for the key
+    /// exist.
+    ///
+    /// Returns all values previously corresponding to the key, if any.
+    ///
+    /// Fails with `header-error.immutable` if the `fields` are immutable.
+    ///
+    /// Fails with `header-error.invalid-syntax` if the `field-key` is
+    /// syntactically invalid.
+    get-and-delete: func(name: field-key) -> result<list<field-value>, header-error>;
+
     /// Append a value for a key. Does not change or delete any existing
     /// values for that key.
     ///