1.12.1 - Third party support (fixed)

Adds base functionality for autocompletion support in third party text areas added by other extensions or scripts.
Includes https://github.com/toshiaki1729/stable-diffusion-webui-dataset-tag-editor so far.
Adding new third party options is easily achieved by adding them in the thirdParty dictionary of the new _textAreas.js file, with their respective IDs or captions.
Feel free to open issues or pull requests for additional third party support.