[api-minor] Combine the textContent/textContentStream parameters

Rather than handling these parameters separately, which is a left-over from back when streaming of textContent was originally added, we can simply pass either data directly to the `TextLayer` and let it handle things accordingly.

Also, improves a few JSDoc comments and `typedef`-imports.

test/driver.js

@@ -263,7 +263,7 @@ class Rasterize {
 
       // Rendering text layer as HTML.
       const task = renderTextLayer({
-        textContent,
+        textContentSource: textContent,
         container: div,
         viewport,
       });

test/unit/text_layer_spec.js

@@ -33,7 +33,7 @@ describe("textLayer", function () {
     const textContentItemsStr = [];
 
     const textLayerRenderTask = renderTextLayer({
-      textContentStream: page.streamTextContent(),
+      textContentSource: page.streamTextContent(),
       container: document.createElement("div"),
       viewport: page.getViewport(),
       textContentItemsStr,