Build/Test Tools: Update terser-webpack-plugin.
This bumps the `terser-webpack-plugin` dependency to version `5.3.9`, which fixes an error during the minification process of the `@wordpress/block-library` package during builds.
Props clorith, azaozz, desrosj.
Fixes #58660, See #57657.
Built from https://develop.svn.wordpress.org/trunk@56097
git-svn-id: http://core.svn.wordpress.org/trunk@55609 1a063a9b-81f0-0310-95a4-ce76da25c4cd
