Addon-docs: Snapshot testing and bug reporting setup for props tables

[shilman]

After launching props table support in @storybook/addon-docs, we've found an impossibly large number of cases to handle (nearly 100 issues so far), many of which correspond to failures in an underlying library.

To properly support prop tables, we need a proper testing and bug reporting strategy. I propose:

• Test suites for each supported framework (react, vue, angular, ember, wc)
• Snapshot the output of the underlying library
• Snapshot the output of our props extraction
• Display the resulting prop table in storybook
• Process for filing issues
• Sweep of existing issues to add them or de-dupe them

This will also help as we move to react-docgen for typescript prop tables in 6.0. #9837

[shilman]

Whoopee!! I just released https://github.com/storybookjs/storybook/releases/tag/v6.0.0-alpha.13 containing PR #9838 that references this issue. Upgrade today to try it out!

You can find this prerelease on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.