js.Build: Add SourceMap flag into js.Build opts which can turn on sourcemap

Add documentation for sourceMap opt in js.Build. Comment on future external support.
Temporarily tagged with 0.75-DEV

content/en/hugo-pipes/js.md

@@ -70,6 +70,10 @@ With the above, these imports should work in both scenarios:
 import * as React from 'react'
 import * as ReactDOM from 'react-dom';
 ```
+sourceMap [string, bool] {{< new-in "0.75-DEV" >}}
+: Let `js.Build` output sourceMap. Current only inline is supported. true defaults to inline.
+  One of: true, false, `inline`, `external`
+  Default is false
 
 target [string]
 : The language target.