webpack-config: Default to 'source-map' in dev mode (#33924)

It seems our developers have spoken: 25 webpack configs in the monorepo
have been setting webpack's `devtool` to 'source-map' for development
builds, while only 8 have used the default from webpack-config (and 2
are explicitly setting no devtool).

So let's change the default to 'source-map' so everything can be working
the same way.

Note this doesn't change anything for production builds, this is only
for development.

Committed via a GitHub action: https://github.com/Automattic/jetpack/actions/runs/6730930552

composer.json

@@ -4,7 +4,7 @@
 	"type": "jetpack-library",
 	"license": "GPL-2.0-or-later",
 	"require": {
-		"automattic/jetpack-assets": "^1.18.13",
+		"automattic/jetpack-assets": "^1.18.14-alpha",
 		"automattic/jetpack-connection": "^1.58.2",
 		"automattic/jetpack-constants": "^1.6.23",
 		"automattic/jetpack-plans": "^0.3.5",