* [vcpkg docs] add docs for manifest files
These are just for the maintainer docs,
not user docs.
* [vcpkg] EBNF-ify platform expression parsing
this modifies nothing about what strings are accepted or rejected,
it just moves stuff around.
also adds tests.
* [vcpkg docs] add manifest mode example
* [wip] docs for augustin
also fix tabs
* [vcpkg manifest] switch to using maps for features
* Apply suggestions from code review
* un-experimentize format-manifest
* flesh out the user manifest mode docs
* CRs
* billy CRs
* final personal pass-thru
