docs: update snowpack-plugin.rst on dynamic imports (#1026)

docs/ref/snowpack-plugin.rst

@@ -5,7 +5,7 @@ API Reference - Snowpack Plugin (@lingui/snowpack-plugin)
 It's a good practice to use compiled message catalogs during development. However,
 running :cli:`compile` everytime messages are changed soon becomes tedious.
 
-``@lingui/snowpack-plugin`` is a snowpack loader, which compiles messages on the fly:
+``@lingui/snowpack-plugin`` is a Snowpack plugin, which compiles messages on the fly:
 
 Installation
 ============
@@ -32,15 +32,30 @@ Simply add ``@lingui/snowpack-plugin`` inside your ``snowpack.config.js``:
       ],
    }
 
-Then in your code all you need is to use dynamic() import. Extension is mandatory. In case of using po format, use ``.po``.
+Then in your code all you need is to use `dynamic imports <https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#dynamic_imports>`_ to load only necessary catalog. Extension is mandatory. In case of using po format, use ``.po``.
 
-.. code-block:: jsx
+.. code-block:: ts
 
    export async function dynamicActivate(locale: string) {
-      const { messages } = await import(`./locales/${locale}/messages.po`)
-      i18n.load(locale, messages)
+      let catalog: {messages: Messages}
+      
+      switch (locale) {
+        case 'cs':
+          catalog = await import('./locales/cs/messages.po')
+          break
+        case 'en':
+        default:
+          catalog = await import('./locales/en/messages.po')
+          break;
+      }
+      
+      i18n.load(locale, catalog.messages)
       i18n.activate(locale)
    }
 
+.. note::
+   Comparing to `Webpack instructions for dynamic loading <./loader.html>`_, code snippet above doesn't rely on variable ``locale`` to do the actual import. Instead, we manually check every possible value using ``switch/case`` and import final catalog by exact path. This is default behavior (or restriction?) of `esbuild <https://esbuild.github.io>`_ - *extremely fast JavaScript bundler* used by Snowpack under the hood. There is `an issue regarding this feature <https://github.com/evanw/esbuild/issues/700>`_
+   Similar restrictions apply to Babel macros or other non-standard features - they won't work with ``esbuild``
+
 See the `guide about dynamic loading catalogs <../guides/dynamic-loading-catalogs.html>`_
 for more info.