Allow user to specify the number of rows or columns to increase or decrease by when resizing pane via keyboard.

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# Description of the new feature/enhancement

When resizing panes via keyboard, it would be nice to specify the resize amount. Right now, when you resize the pane, the amount is different depending on pane width, window width, etc. Can we have the ability to resize by X columns or rows (depending on resize direction)?

# Proposed technical implementation details (optional)

In the keyboard settings, when assigning a keyboard shortcut for pane resizing, allow a second integer value specifying the number of columns or rows to resize by.

If the user specifies the value 2 for the resize pane action, then if they grow or shrink the pane, it will increase or decrease by 2 rows or columns depending on direction.

Start by tracing the keyboard settings and pane-resize action in the Windows Terminal codebase; the issue does not name specific files or tests. Define how the optional integer is represented and applied to row or column resizing, then verify that configured resize shortcuts change panes by the requested amount.

Written by the indexing model from the issue text.