update rule syntax to support scope specification for dynamic analysis flavor

[williballenthin]

as described in #1517 (comment), update the rule syntax to support the rule metadata structure like:

rule:
  meta:
    name: create file
    scope: 
      - static: function
      - dynamic: thread
  features:
    or:
      api: CreateFile
      api: fopen

this will require changes in a bunch of places, including:

• documentation
• yaml parser/validator
• result document metadata structure
• serializer/deserializer

this issue doesn't describe how capa uses the scope specifier - only that it can parse rules with this new syntax.

while this feature is in development, its ok to map rules with rule.meta.scope: XXX to rule.meta.scope.static: XXX and rule.meta.scope.dynamic: thread. we can do a cleanup of the capa-rules once this feature works well.

the PR should be accompanied by test cases that show what happen when invalid syntax is encountered, such as unknown scope names.

[yelhamer]

This feature has been implemented in v7 of capa :)