Replace Activity Bar sizing presets with a numeric size setting

## What
VSCode would benefit from replacing the current Default/Compact Activity Bar sizing modes with a numeric input, preferably specifying icon size in pixels or another unit.

Users could enter a value such as 20px, instead of being limited to `Default`’s 24px or `Compact`’s 16px. (These pixel values don't appear to be that different on paper, but in practice it feels like `Compact` is half the size of `Default`)

## Why
This would improve accessibility, especially for users with low vision, as they could configure larger controls, while allowing power users to choose smaller or intermediate sizes.

The two current size modes are both arbitrary in their sizing and limited in their variety. This is a small but impactful UI change that, in my opinion, is in the spirit of this line from the `Wiki`:

> Since we want to deliver a lightweight code editor, the UX should feel lightweight as well and not be cluttered.

## How
Existing preferences should migrate to equivalent numeric values so users retain their current appearance - it is trivial to implement this change in a way that can allow for backward and forward compatibility as the project desires.

### I’d like to contribute an implementation. Would you welcome a PR for this change?