Wrapper function for creating plugins

Another v5 feature we should consider is tweaking the docisfy plugin architecture to be more resilient. This warrants further discussion, but here are two ideas worth considering:

1. Provide a docisfy plugin "starter" or a sample repo to use as a starting point.

This would make life much easier for the community and help enforce acceptance criters (below).

1. Define acceptance criteria for all docsify plugins.

This is critical for docsify because unlike static sites, a docsify-powered site requires Javascript to render content. If a docsify plugin throws an error, it is almost certain that site functionality will be broken.

Here are a few idea for acceptance criteria:

- Specify the version of ECMAScript plugin developers should target. For docsify v4.x, ES5 is the required target to ensure IE11 support. This means dev must either author their plugin with ES5 syntax or transpile their code from ES6+ to ES5. For docsify v5, we'll have to look at our supported browser list to determine the appropriate ECMAScript version to target.
- Plugin code must be wrapped in a `try { ... } catch(err) { ...}` statement to prevent a plugin error from breaking a site.
- Plugins must make their version number accessible to other plugins for interoperability. Docsify addons like [docsify-themeable](https://jhildenbiddle.github.io/docsify-themeable/) and [docsify-tabs](https://jhildenbiddle.github.io/docsify-tabs/) already do this by adding an object to the global `$doscify` object (`$docsify.tabs` and `$docsify.themeable`) with a `value` key and value. The goal is to allow both docsify and other plugins to determine which plugins are active and, if necessary, modify their behavior as needed.
- Plugins must specify a compatible docsify version to prevent breaking sites when a new version of docsify is released. For example, consider a v4 site that is configured to load the latest version of docsify and multiple plugins. If docsify v5 is released using the same URLs, the v4 site will automatically be "upgraded" to v5 but will likely break as a result of trying to load v4 plugins. This safeguard would allow docsify (or the plugin itself) to determine if it is safe to run the plugin before doing so, which would at least allow the site to load and function properly (albeit without incompatible plugin functionality). This wouldn't be necessary if users locked their docsify, theme, and plugin CDN URLs to a major version (see #780), but since they don't a safeguard like this is necessary.

Just some ideas to get started.

_Originally posted by @jhildenbiddle in https://github.com/docsifyjs/docsify/issues/1061#issuecomment-641731543_