Add nightly-only error on ImportType resolution mode assertion

Author: andrewbranch

src/compiler/checker.ts

@@ -36029,6 +36029,9 @@ namespace ts {
             if (node.assertions) {
                 const override = getResolutionModeOverrideForClause(node.assertions.assertClause, grammarErrorOnNode);
                 if (override) {
+                    if (!isNightly()) {
+                        grammarErrorOnNode(node.assertions.assertClause, Diagnostics.Resolution_mode_assertions_are_unstable_Use_nightly_TypeScript_to_silence_this_error_Try_updating_with_npm_install_D_typescript_next);
+                    }
                     if (getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.Node16 && getEmitModuleResolutionKind(compilerOptions) !== ModuleResolutionKind.NodeNext) {
                         grammarErrorOnNode(node.assertions.assertClause, Diagnostics.Resolution_modes_are_only_supported_when_moduleResolution_is_node16_or_nodenext);
                     }