Set extensions as preview

Remove this when we are ready to publish the official versions.

packages/salesforcedx-vscode-apex/package.json

@@ -1,4 +1,5 @@
 {
+  "preview": true,
   "name": "salesforcedx-vscode-apex",
   "displayName": "Apex Code Editor for Visual Studio Code",
   "description": "Provides code-completion for the Apex programming language",

packages/salesforcedx-vscode-core/package.json

@@ -1,4 +1,5 @@
 {
+  "preview": true,
   "name": "salesforcedx-vscode-core",
   "displayName": "SFDX CLI integration for Visual Studio Code",
   "description": "Provides integration with the SFDX CLI",