Add a custom build for swift-docc-render

[matthewbastien]

We need to build a special copy of swiftlang/swift-docc-render that will work in VS Code. However, this build does not need to happen every single time we build the extension. So, as a nice middle-ground I've opted to create a NPM script called update-swift-docc-render that will:

1. clone swift-docc-render into a temporary directory
2. apply our custom patches to swift-docc-render
3. build a copy of swift-docc-render into assets/swift-docc-render

The asset will be checked into source control so that we don't have to build it unless swift-docc-render changes in the future.

[matthewbastien]

It's also worth noting that building swift-docc-render requires node v18 which is a couple versions behind the version that VS Code uses.

[matthewbastien]

Needed for #562

[award999]

@swift-ci Please test