Add readonly modifier to DOM declarations

[mhegazy]

This is gated on: microsoft/TypeScript#6532

The webidl file already provide this information, we need to translate that in the script:

for instance read-only:
https://github.com/Microsoft/TSJS-lib-generator/blob/ea717dddb01dbeb6386b844f667194f71c7d7165/inputfiles/browser.webidl.xml#L408

and constant : https://github.com/Microsoft/TSJS-lib-generator/blob/ea717dddb01dbeb6386b844f667194f71c7d7165/inputfiles/browser.webidl.xml#L315