Move TinyMCE code into its own module

[GuySartorelli]

TinyMCE 7 changed its license to GPLv2+, which isn't compatible with our licenses, so we need to swap to a new editor.

Regardless of whether a new editor is available in CMS 6.0.0 or in a future minor release, we want to:

1. Make sure TinyMCE 6 is still available for people who aren't able to swap away from it immediately
2. Move the TinyMCE 6 code into its own module that can be included or excluded from projects easily.

Acceptance Criteria

The below all apply for CMS 6 only

• All TinyMCE related code (JavaScript, PHP, YAML, localisations, etc) from all supported modules is moved into a single new repository
• The default "cms" editor config is provided in the new module
• TinyMCE plugins that rely on a specific module (e.g. "link to a page") are only applied if the corresponding module is installed
• There is configuration to disable the "cms" editor config from being set (e.g. if someone has two editor modules installed, they can pick which one controls the "cms" editor config with a simply YML config variable)
• When this module is installed, the behaviour is identical to the CMS 5 behaviour
• This module is installed as part of silverstripe/installer by default for CMS 6