Add a parser type `text` to WebsiteAgent.

Sometimes you need to parse a document that has no structure or is simply not HTML/XML or JSON.
This can be useful to check updates on a plain text, source file, CSV/TSV, etc.
No, `/./` does not match a newline without the `//m` flag.  There are several ways to match "any character including newline":

- /(?:.|\n)/
- /(?m:.)/
- /[\S\s]/
- /\p{Any}/

etc.

I thought the first one is most straightforward and understandable to anyone.