chore: Add @metamask-previews/* to NPM age gate exceptions (#3699)

We sometimes use `@metamask-previews/*` to use preview builds for
testing certain features. These will usually not be 3 days old, so can't
be installed without an exception.

<!-- CURSOR_SUMMARY -->
---

> [!NOTE]
> Adds `@metamask-previews/*` to `npmPreapprovedPackages` in
`.yarnrc.yml` to bypass the 3-day NPM minimal age gate.
> 
> <sup>Written by [Cursor
Bugbot](https://cursor.com/dashboard?tab=bugbot) for commit
571c068. This will update automatically
on new commits. Configure
[here](https://cursor.com/dashboard?tab=bugbot).</sup>
<!-- /CURSOR_SUMMARY -->

Author: Mrtenz

.yarnrc.yml

@@ -28,4 +28,5 @@ npmMinimalAgeGate: 4320 # 3 days (in minutes)
 # regardless of their publish age.
 npmPreapprovedPackages:
   - '@metamask/*'
+  - '@metamask-previews/*'
   - '@lavamoat/*'