API tweaks for grid editor

[bpasero]

@jrieken if you could look at the vscode.d.ts API changes (will also present in todays API call).

Summary of changes:

• new ViewColumn.BESIDE to open an editor to the side of the active one
• ViewColumn goes all the way to ViewColumn.Nine
• textEditor.viewColumn will be a number even if viewColumn > ViewColumn.Nine to ensure any checks extensions do for this value being undefined is not broken in grid
• mapping of ViewColumn to grid is done by respecting the order of editors as they appear in the grid (ViewColumn.ONE will always be the very first editor, ViewColumn.TWO the next one, etc.)
• new vscode.setEditorLayout command to set an editor layout from an extension using the syntax we already have in the workbench (e.g. for a 2x2 grid: { groups: [{ groups: [{}, {}], size: 0.5 }, { groups: [{}, {}], size: 0.5 }] }

[bpasero]

/cc @mjbvz if this has any impact for the webview, but it should not I think.

The new ViewColumn.Beside makes the code much nicer in https://github.com/Microsoft/vscode/blob/5399d6308883664eb2833458d150445f3d19ffb6/extensions/markdown-language-features/src/commands/showPreview.ts#L43 (via 5399d63)

[eamodio]

@bpasero rather than Beside (which I'm assuming will open to the right of the active editor?), what do you think about maybe having Before and After (or Left and Right)? To make it easy for an extension to chose how it should be positioned with respect to the active editor?

[bpasero]

@eamodio I do not think there is a use case for it for most extensions so I would stay away from exposing this as real API. You can always trigger one of the workbench commands to get this behaviour (workbench.action.newGroupLeft, workbench.action.newGroupRight, workbench.action.newGroupAbove, workbench.action.newGroupBelow) and then just open an editor normally.

ViewColumn.BESIDE will open on the right hand side of the active editor but on the bottom side if the user changed the workbench.editor.openSideBySideDirection setting to down.