feat: allow FileContentSetter to return string or SourceDescription

- Update `FileContentSetter` type to include string as a possible return value
- Modify `getFileContent` function to handle both string and SourceDescription returns
- Update README.md to reflect the new return type in the documentation

Author: lingbopro

README.md

@@ -59,7 +59,7 @@ export type FileContentSpecifier = string | FileContentSetter;
 export type FileContentSetter = (
   id: string,
   originalCode: string
-) => SourceDescription;
+) => string | SourceDescription;
 ```
 
 details:
@@ -93,12 +93,12 @@ Values are handled in the same way as [`include`](#include)
 
 ### `content`
 
-Type: `String` | `(id: string, originalCode: string) => SourceDescription`  
+Type: `String` | `(id: string, originalCode: string) => string | SourceDescription`  
 **Mandatory**
 
 Specifies the content of the imported module.
 
-For functions, the string value returned by the function will be passed to Rollup to change the content of the file
+For functions, the string or SourceDescription value returned by the function will be passed to Rollup to change the content of the file
 The [plugin context](https://rollupjs.org/plugin-development/#plugin-context) will be `this` for the function
 
 > `SourceDescription` is a rollup interface (see more in [Rollup Docs](https://rollupjs.org/plugin-development/#transform)):

src/file-content.ts

@@ -17,4 +17,4 @@ export type FileContentSpecifier = string | FileContentSetter;
 export type FileContentSetter = (
   id: string,
   originalCode: string
-) => SourceDescription;
+) => string | SourceDescription;

src/utils.ts

@@ -47,15 +47,20 @@ export function getFileContent<self>(
     };
   } else if (typeof specifier === 'function') {
     const ret = specifier.call(_this, id, originalCode);
-    if (typeof ret === 'object') {
+    if (typeof ret === 'string') {
+      return {
+        code: ret,
+        map: null,
+      };
+    } else if (typeof ret === 'object') {
       if (typeof ret.code !== 'string')
         throw new TypeError(
           'The returned SourceDescription of the content function must have a code property'
         );
       return ret;
     } else
       throw new TypeError(
-        'The return value of the content function must be SourceDescription'
+        'The return value of the content function must be string | SourceDescription'
       );
   } else
     throw new TypeError(