Add plugin apis to control spec design functionality

Is your feature request related to a problem? Please describe. As a plugin author it would be nice to be able to have APIs for the design panes to hook into the design process. Describe the solution you'd like • Being able to control the language parser for the spec loaded • Being able to hook into the spec loaded event • Being able to hook into spec change event • Being able to control the linter output • Being able to control the validation errors • Being able to control the sidebar output for a spec • Being able to control the preview pane output for a spec • Being able to hook into the tab change event (design -> debug) and override the request generation Additional context I may have missed some events here, but ideally, whatever would be required to move openapi support into a plugin would be evented and hooked 🙌 By providing above, we could move OpenAPI support to a plugin and allow plugin authors…