js: Add Inject config option

Fixes #8164

content/en/hugo-pipes/js.md

@@ -43,6 +43,9 @@ minify [bool]
 avoidTDZ {{< new-in "0.78.0" >}}
 : There is/was a bug in WebKit with severe performance issue with the tracking of TDZ checks in JavaScriptCore. Enabling this flag removes the TDZ and `const` assignment checks and may improve performance of larger JS codebases until the WebKit fix is in widespread use. See https://bugs.webkit.org/show_bug.cgi?id=199866
 
+inject [slice] {{< new-in "0.81.0" >}}
+: This option allows you to automatically replace a global variable with an import from another file. The path names must be relative to `assets`.  See https://esbuild.github.io/api/#inject
+
 shims {{< new-in "0.81.0" >}}
 : This option allows swapping out a component with another. A common use case is to load dependencies like React from a CDN  (with _shims_) when in production, but running with the full bundled `node_modules` dependency during development: