Drop deprecated end points, members, options & defaults

[eemeli]

As a first step towards 2.0, let's break everything. Or at least the stuff that was already deprecated.

• The following entry points are removed:
  • 'yaml/map'
  • 'yaml/pair'
  • 'yaml/scalar'
  • 'yaml/schema'
  • 'yaml/seq'
  • 'yaml/types/binary'
  • 'yaml/types/omap'
  • 'yaml/types/pairs'
  • 'yaml/types/set'
  • 'yaml/types/timestamp'
• The package.json "exports" no longer include an "./": "./" escape hatch.
• Schema.defaultTags and Schema.defaultPrefix are removed.
• The tags alias for the customTags option is removed, and prettyErrors is now default-true.
• Custom tags that use class instead of identify() are no longer supported.