Splitter base

[MayaKirova]

Related to #6639

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code
• This PR includes API docs for newly added methods/properties
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes
• This PR includes behavioral changes and the feature specification has been updated with them

[3phase]

After testing the keyboard navigation PR, there seemed to be a weird behavior when moving the right border of the central grid pane a little bit to the left and then using Ctrl+Left Arrow to hide the whole central pane. The second misbehavior is when focusing the left border of the central pane via Shift+Tab and clicking Ctrl+Right Arrow. The rightmost grid won't get expanded at its maximal width, instead would stay cut. My explanations are described better by the gif below.