Make --include-extended-types default for Swift 5.9 (swiftlang#57)

* make --include-extended-types default for Swift 5.9 and introduce --exclude-extended-types flag

* address review comments
 - mention default behavior earlier in docs
 - be more precise about method/function terminology in docs

Add experimental flag for skipping synthesized symbols (swiftlang#28)

Adds support for the generation of DocC archives without synthesized
symbols by passing a new, experimental, plugin flag
`--experimental-skip-synthesized-symbols`.

Enabling this flag does not transform or post-process any of the passed
arguments but overrides the default value of
`symbolGraphOptions.includeSynthesized` that is provided to SwiftPM to false.

Tag version 1.1.0

This time, with new upstream SymbolKit version 1.0.0.

Initial release.