fix: correct behavior of plugins retrieval functions

[jmakhack]

Related Issue

Closes #1308

Your solution

Changed the byType method to no longer accept undefined as a valid type. Added the all method to PluginsContainer as the correct way to retrieve all plugins.

How Has This Been Tested?

I ran my changes against the existing tests, making sure that plugins.test.js, modelFields.test.js, validators.test.js, and contentModelToSDL.test.js passed in particular.

I also checked all uses of the byType method within the codebase to ensure that it was never being passed a value of undefined as a means for retrieving all plugins. As a result of this check, I updated the getPlugins function in index.ts to return all plugins when passed a value of undefined. This is expected behavior based on the tests found in plugins.test.js.

Screenshots (if relevant):

N/A

[brunozoric]

Everything looks good :)