tamer: nir::air: Include mention of .experimental file in TODO help

The previous commit introduced support for a `.experimental` file to tigger
`xmlo-experimental`.  This modifies the error message for unsupported
features to make mention of it to help to the user track down the problem.

DEV-13162

tamer/src/nir/air.rs

@@ -398,6 +398,10 @@ impl Diagnostic for NirToAirError {
                         are you unintentionally using the `--emit xmlo-experimental` \
                         command line option?"
                 ),
+                tok.span().help(
+                    "this package may also have a sibling `.experimental` file \
+                        that triggers `xmlo-experimental`"
+                ),
             ],
 
             MatchSubjectExpected(ospan, given) => vec![