Fix serialization and mutation of <textarea> elements (#1351)

* Fix serialization and mutation of <textarea> elements taking account the duality that the value can be set in either the child node, or in the value _parameter_ (not attribute)

* Backwards compatibility: Bug fix and regression test for #112
 - this is to fix up 'historical' recordings, as duplicate textarea content should no longer be being created at record time
 - new test shows what the snapshot generated by previous versions of rrweb used to look like, hence 'bad'
 - original 0efe23f04a fix either didn't work or no longer works due to childNodes being appended subsequent to this part of the code
 - during review, we also verified that the `_cssText` case should still be handled okay, as there's currently no scenario where csstext is present with css child nodes of a <style>

* Masking: Fix that textarea values were being missed by the masking system if the value was recorded as a child node
 - I didn't notice that form.html was used in other tests, so lucky that I noticed that those tests also should have the 'pre value' masked out

* Simplify by always storing the textarea value in the `.value` attribute (from it's DOM property) and not as a childNode. It should still be rebuilt as a childNode rather than a property
---------

Authored-by: eoghanmurray <eoghan@getthere.ie>

packages/rrweb-snapshot/test/__snapshots__/integration.test.ts.snap

@@ -247,6 +247,7 @@ exports[`integration tests [html file]: form-fields.html 1`] = `
       </label>
       <label for=\\"textarea\\">
         <textarea name=\\"\\" id=\\"\\" cols=\\"30\\" rows=\\"10\\">1234</textarea>
+        <textarea name=\\"\\" id=\\"\\" cols=\\"30\\" rows=\\"10\\">1234</textarea>
       </label>
       <label for=\\"select\\">
         <select name=\\"\\" id=\\"\\" value=\\"2\\">