Add textResolver to preprocess text values

[edvinasjurele]

Context

There might be some cases which requires controlling the text values in text nodes. Some of the use cases are:

• translations (register/translate text value via custom admin solution, f.e. adding ICU message syntax support)
• text sanitisation, encoding/decoding
• text replacement (f.e. have some replacable placeholders like {date} so the app depending on business logic would inject the correct)

TODO

• Add textResolver to support preprocessing of the text values in rich text
• Add DEMO example
• Update README about the possibility of such functionality
• Cover with TS types