Make Opal compatible for JS usage

[hmdne]

From my initial outlook, this will involve porting the code:

• to not use mutable strings
• to be able to work with Oga in addition to Nokogiri (https://github.com/metanorma/coradoc/issues/90)

Other than that, the work in the Plurimath project is already done on making the dependencies work.

If the second task is to be done by creating a compatibility layer, my suggestion would be to make this compatibility layer conform to the API of Nokogiri as much as possible. This approach has been done by at least two libraries that I know of:

• opal-browser
• ferrum

[ronaldtse]

It sounds worthwhile, to develop an XML/HTML adapter layer, using Nokogiri syntax that is Opal compatible. That way, all our projects can share this.