refactor: move emit from callback to helper file function (#704)

* Change emit from callback to helpers file function

* Update docs, remove commented code, improve type sig

docs/advanced/making plugins.md

@@ -216,22 +216,19 @@ export type QuartzEmitterPlugin<Options extends OptionType = undefined> = (
 
 export type QuartzEmitterPluginInstance = {
   name: string
-  emit(
-    ctx: BuildCtx,
-    content: ProcessedContent[],
-    resources: StaticResources,
-    emitCallback: EmitCallback,
-  ): Promise<FilePath[]>
+  emit(ctx: BuildCtx, content: ProcessedContent[], resources: StaticResources): Promise<FilePath[]>
   getQuartzComponents(ctx: BuildCtx): QuartzComponent[]
 }
 ```
 
-An emitter plugin must define a `name` field an `emit` function and a `getQuartzComponents` function. `emit` is responsible for looking at all the parsed and filtered content and then appropriately creating files and returning a list of paths to files the plugin created.
+An emitter plugin must define a `name` field, an `emit` function, and a `getQuartzComponents` function. `emit` is responsible for looking at all the parsed and filtered content and then appropriately creating files and returning a list of paths to files the plugin created.
 
-Creating new files can be done via regular Node [fs module](https://nodejs.org/api/fs.html) (i.e. `fs.cp` or `fs.writeFile`) or via the `emitCallback` if you are creating files that contain text. The `emitCallback` function is the 4th argument of the emit function. Its interface looks something like this:
+Creating new files can be done via regular Node [fs module](https://nodejs.org/api/fs.html) (i.e. `fs.cp` or `fs.writeFile`) or via the `write` function in `quartz/plugins/emitters/helpers.ts` if you are creating files that contain text. `write` has the following signature:
 
 ```ts
-export type EmitCallback = (data: {
+export type WriteOptions = (data: {
+  // the build context
+  ctx: BuildCtx
   // the name of the file to emit (not including the file extension)
   slug: ServerSlug
   // the file extension