src/goLanguageServer: use isInPreviewMode to enable experiments

Previously, we depended on the extensionID to enable experimental
features in golang.go-nightly extensions. Now, we enable all the
experimental features if the extension is in preview mode even when
the extension name is golang.go (built from master).

Change-Id: I36b8ed5d1d9cdc0a003ec2dab911fe4a8e844284
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/289190
Trust: Hyang-Ah Hana Kim <hyangah@gmail.com>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Peter Weinberger <pjw@google.com>
diff --git a/src/goLanguageServer.ts b/src/goLanguageServer.ts
index 0a7cf93..fa57f3f 100644
--- a/src/goLanguageServer.ts
+++ b/src/goLanguageServer.ts
@@ -583,7 +583,7 @@
 	workspaceConfig = passGoConfigToGoplsConfigValues(workspaceConfig, getGoConfig(resource));
 
 	// Only modify the user's configurations for the Nightly.
-	if (extensionId !== 'golang.go-nightly') {
+	if (!isInPreviewMode()) {
 		return workspaceConfig;
 	}
 	// allExperiments is only available with gopls/v0.5.2 and above.