Move 2-step caret movement to -typing and refactor

[tomalec]

This is a result of a meeting after the review of #7356

📝 Provide a description of the improvement

Changes from #7356 shown that current implementation of 2SCM does not work well with multiple attributes, and needs some capability to synchronize gravity between attributes. At the F2F with @Reinmar, @oleq and @niegowski we agreed it requires some refactoring.

We should start with/consider:

1. moving it to -typing,
2. make a plugin,
3. Consider having the gravity set per attribute.

We also agreed, there should be only 2 movements required for multiple attributes ending in the same position. E.g. "some linking code[]" + ←←, results in "some linking code[]". Meaning stepping inside steps into the deepest attribute at given position. It is to minimize the overhead of additional ←s needed for navigation, simplify the behavior and implementation, especially given, looking at the model with a flat structure of attributes, it's hard to identify the depth of nesting.

📃 Other details

• Browser: n/a
• OS: n/a
• CKEditor version: master @
• Installed CKEditor plugins: link, code, engine

Related issues

• Rethink two-step caret movement #4341 - idea to rething 2SCM,
• Allow distinguishing between "cursor inside/outside inline code" (same as for links) #6722 - add 2SCM to the code,
• Two-step caret movement doesn't work next to <br> #4361 - 2SCM doesn't work with <br>,
• Overriding selection gravity breaks keyboard navigation in RTL content #4528 - overriding gravity breaks RTL nav

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