[MAINTENANCE] Improve react-docgen usage

[Alex Johnson]

Right now, component libraries need to include react-docgen specified as a devdep, and it's stuck on the 5.x series even though react-docgen itself is up to 7.x, because it's actually used by extract-meta.js in this repo, via the dash-generate-components command. Can we either:

• Use a version of react-docgen provided by Dash - if we did this we would either need to include a built react-docgen with dash, or have dash-generate-components run npm i inside Dash, since component authors would not generally have built Dash itself.
• At least update extract-meta.js (and dash-generate-components if necessary) to support react-docgen 7.x (but maintaining support for 5.x for compatibility with existing components). Not ideal, since components have no direct dependency on react-docgen, but at least component authors could keep their dependencies up to date.

Also note: It looks like Typescript component generation pulls in Typescript from the component's node_modules, and this should not change. Typescript projects are built with a specific Typescript version so should be parsed with that same version. This means we'll need to keep extract-meta.js working with any changes that might be introduced in Typescript, but that seems unavoidable (and so far hasn't been an issue 🙏)

@T4rk1n curious your thoughts.