docs: Adds notes to Superset plugin generator about TypeScript errors

[alex-bain]

SUMMARY

When creating a new Superset plugin, the generator creates a tsconfig.json file. When trying to build Superset with this new plugin, the build fails due to TypeScript compilation errors.

This PR extends the README documentation of a generated plugin to call out some of the changes a user may want to make to fix these TypeScript errors. These suggested changes already exist in the echarts plugins, and served as inspiration for how to fix the TypeScript errors I encountered in my own plugin.

BEFORE/AFTER SCREENSHOTS OR ANIMATED GIF

Screenshots of TypeScript errors after creating the plugin:

TESTING INSTRUCTIONS

• Use the generator to create a new Superset plugin in the superset-frontend/plugins/ directory alongside other plugins
• Follow the final manual steps to add this plugin to MainPreset.js and package.json
• Try to build Superset
• Notice that there are TypeScript errors such as unresolvable modules (@superset-ui/core)
• Notice that there are TypeScript errors such as unresolved Superset types
• Make the new changes suggested in the plugin README file
• Notice that the errors are resolved and the plugin will build successfully

[github-actions]

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[villebro]

Thanks for these improvements @alex-bain !