Refactor parser into a page instance variable

[ang-zeyu]

What is the purpose of this pull request? (put "X" next to an item, remove the rest)

• [x] Other, please explain: small code refactor, originally part of #1189 ( decided to extract this out as well so less changes are observed over there )

What is the rationale for this request?
We use parser ( markbinder ) in several places in Page.js now. Passing it around as a parameter isn't really ideal (#1225), hence, let's make it an instance variable instead.

What changes did you make? (Give an overview)

• change markbinder variable name to parser
• extract parser as an instance variable, and tweak the logic for creating the resolveDependency promises accordingly.
• remove includedFiles collection in resolveDependency ( we can just collect it once in page.generate() now, since there's only one parser )

Is there anything you'd like reviewers to focus on?
na

Testing instructions:
npm run test should pass

Proposed commit message: (wrap lines at 72 characters)
Refactor parser into a page instance variable

[ang-zeyu]

shelving this for now, although not as clean, realised there's some benefits to constructing a new instance for each generation, especially with regards to plugin hooks. Perhaps a better solution will soon be found.