docs(plugins): add parser.preprocess (#4365)

* docs(plugins): add parser.preprocess

* docs: fix linting

docs/plugins.md

@@ -87,7 +87,8 @@ export const parsers = {
     astFormat: "dance-ast",
     hasPragma,
     locStart,
-    locEnd
+    locEnd,
+    preprocess
   }
 };
 ```
@@ -104,12 +105,18 @@ The location extraction functions (`locStart` and `locEnd`) return the starting
 function locStart(node: object): number;
 ```
 
-The pragma detection function (`hasPragma`) should return if the text contains the pragma comment.
+_(Optional)_ The pragma detection function (`hasPragma`) should return if the text contains the pragma comment.
 
 ```ts
 function hasPragma(text: string): boolean;
 ```
 
+_(Optional)_ The preprocess function can process the input text before passing into `parse` function.
+
+```ts
+function preprocess(text: string, options: object): string;
+```
+
 ### `printers`
 
 Printers convert ASTs into a Prettier intermediate representation, also known as a Doc.