Add Open Active Diff Side option to Command Palette using DIFF_OPEN_SIDE command

[LeftPhalange]

Addresses issue #260762. This adds the workbench.action.compareEditor.openSide or DIFF_OPEN_SIDE constant in to the MenuRegistry, which registers it into the Command Palette.

Below is a screenshot showing it in the command palette.

How to Test

1. Open Visual Studio Code.
2. Open a folder with two files.
3. Select one file for compare in the Explorer pane by right-clicking it and click Select for Compare in the context menu.
4. Select another file by right-clicking it and clicking on Compare with Selected in the context menu.
5. The diff viewer should now open. Click on one of the file editors. This can be either side.
6. Open the Command Palette by hitting CTRL + SHIFT + P (Command/Cmd for Mac, instead of CTRL).
7. Look for "Open File on Selected Side". Click it while the desired file editor on either side is selected.
8. The desired file should be opened on its own.

[hediet]

Thanks for the PR! Why is the command called Open File on Selected Side though?

[LeftPhalange]

@hediet - good question! When I created this PR, my understanding of the function DIFF_OPEN_SIDE is that the file on the selected side is opened when the chord CTRL + K and SHIFT + O is triggered in the diff editor, per issue #260762. I realize that any side of the editor does not necessarily mean it is a file, but just an editor.

Would other verbiage make more sense for this? For instance:

1. Open Active Side in Editor (Open Active Diff Side in Editor?)
2. Open Active Diff Side
3. Open Selected Side in Editor