fix(dom): Escape ]]> when serializing CharData

[karfau]

to adhere to the XML specification

https://www.w3.org/TR/xml/#NT-CharData
#58 (comment)

[brodybits]

LGTM in general. Unfortunately some of this is very new for me and I am relying on the comments and my understanding of the referenced specs. Thanks for working on this.

[karfau]

I needed to revert the usage of const since we decided to stick to ES5 syntax in lib. Thx eslint for reminding me. (And assigning those values to vars just to access them in a single place doesn't seem to make sense to me.)