[CS2] CSX spread attributes: <div {props…} /> (#4607)

* CSX spread attributes: <div {props...} />

* whitespace cleanup

* Style

* valid CSX attributes

* added comments; cleanup

* Fixed allowed CSX properties.

* Cleanup

* Typo

* Improved RegEx

* Reworked CSX attributes

* small fix for CSX attribute validation

* cleanup

* tests

* fix nested assignement; remove unused variable

* cleanup; improve tests

* fix esoteric case; improve tracking nested splats in CSX tag

Author: zdenko