implement changes discussed in April 20 SWG call and more

[cportele]

• updated requirements so that a Styles resource is not tied to the landing page and can also be, for example, associated with a feature collection resource (closes #1, starts to address #8)
• use of Common Core instead of referencing Features Core
• use of the general req classes Create/Replace/Delete and Update currently in Feature Part 4 as the basis for the req class manage-styles
• use of Prefer header preference "handling" in the req class style-validation
• editorial updates and clarifications
• removed req classes for file resources, this is not really necessary and such a capability can be implemented in various ways without interoperability issues
• removed req classes queryables (no longer needed - tiles and features have mechanisms to describe the schemas of attributes) and style-info (no longer needed)

[jeffharrison]

Styles SWG discussed this merge on 4 May 2021. Update removes Requirements Classes without affecting functionality.

Group discussion on Queryables.

NOTUC to merge pull request.

[jeffharrison]

NOTE: This update will be available for upcoming Tiles, Maps and Styles Sprint later in May.