(maint) Add small helper script to update changelog

This commit adds a new `update.ps1` script to update
the release notes inside the nuspec file to include the
actual changelog.

src/chocolatey-core.extension/update.ps1

@@ -0,0 +1,4 @@
+Import-Module wormies-au-helpers
+
+$changelog = (Get-Content -Encoding UTF8 "$PSScriptRoot\CHANGELOG.md" | Select-Object -Skip 2) -join "`n"
+Update-Metadata -key 'releaseNotes' -value $changelog -NuspecFile "$PSScriptRoot\chocolatey-core.extension.nuspec"