Add new backend for handling MongoDB's textual BSON serialization ("shell mode")

[cowtowncoder]

Since Mongo(DB) defines a set of extensions ("shell mode" -- see https://docs.mongodb.com/manual/reference/mongodb-extended-json/), and since support for reading such "enhanced" content seems sparse, it seems useful to add a backend to support handling it.

Note: this does NOT mean reading actual binary BSON content, for which separate backend is needed: there already exists bson4jackson.

Also: for bonus points we may consider either separate backend (or other type of module... datatype?), or improvement of the initial one, for supporting "strict" mode, which is actually JSON-based and uses structural additions (instead of syntax tweaks) for same purpose.