Tidy up parser

[Rich-Harris]

a few small tweaks:

• read_context is now read_pattern which is a better name
• its return value is typed, providing more type safety
• parser.append calls have been annotated. the annotations look a bit roundabout, but this is the equivalent of doing parser.append<type>(...) which is otherwise impossible in JSDoc. This is better than the previous approach of awkwardly typing the argument — it's safer (it already caught a type error, oops) and doesn't require us to use Omit
• added typeAnnotation to the estree BaseNode

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Latest commit: 0595770

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