Support Run Cells in Section from md cells & context menus

[Yoyokrazy]

Related to: #209653

Main point is to support a context menu addition for md cells, allowing the user to right click and execute the section without leaving the notebook editor.

Currently, to use Run in Section, the user has to do one of the following:

• collapse the entire section, showing the x cells hidden hint with a run button
• scroll it out of view, then right click sticky scroll to execute the section (likely not discoverable to begin with)
• navigate to the outline view (which many have previously hidden due to the previously less useful/overly verbose nature of it)

Potential improvements:

• md cell context menu
• run button for md cells that execute a section
• md cell toolbar (similar to code cell run above/below)
• right click a code cell container to run the section it belongs to (potentially could only show if within a section, avoiding it becoming a fake "run all")
• improving the run in section command to be able to take in the enclosing section of any focused cell (could be used via keyboard shortcut)