docs(externals): document defer/source phase preservation in ESM externals

Webpack 5.107 preserves the defer and source import phase keywords on
external dependencies the same way it preserves import attributes. Adds
subsections under externalsType.module and externalsType.import showing
the static and dynamic forms emitted in the output bundle.

Refs: webpack/webpack#20934

Author: bjohansebas

src/content/configuration/externals.mdx

@@ -489,6 +489,24 @@ jq(".my-element").animate(/* ... */);
 
 Note that there will be an `import` statement in the output bundle.
 
+#### Preserving phase keywords
+
+<Badge text="5.107.0+" />
+
+The `defer` and `source` import phase keywords are preserved on `module` externals the same way [import attributes](/configuration/module/#ruleparserimportattributes) are. A static `import defer * as ns from "mod"` against a `module` external is emitted as a native `import defer * as ...` statement, and `import source v from "mod"` becomes `import source ... from "mod"`. The same external imported with two different phases produces distinct `ExternalModule` instances, so neither phase is silently dropped.
+
+{/* eslint-skip */}
+
+```js
+// input
+import defer * as ns from "external-mod";
+import source v from "external-mod";
+
+// emitted output (with externalsType: "module")
+import defer * as ns from "external-mod";
+import source v from "external-mod";
+```
+
 ### externalsType.import
 
 <Badge text="5.94.0+" />
@@ -536,6 +554,24 @@ async function foo() {
 
 Note that the output bundle will have an `import()` statement.
 
+#### Preserving phase keywords
+
+<Badge text="5.107.0+" />
+
+Dynamic `import.defer(...)` and `import.source(...)` are also preserved on `import` externals when the import function name is the default `"import"`. The phase keyword is emitted in the output instead of being stripped.
+
+{/* eslint-skip */}
+
+```js
+// input
+const ns = await import.defer("external-mod");
+const src = await import.source("external-mod");
+
+// emitted output (with externalsType: "import")
+const ns = await import.defer("external-mod");
+const src = await import.source("external-mod");
+```
+
 ### externalsType.module-import
 
 <Badge text="5.94.0+" />