Introduce setting for font size

<!-- ⚠️⚠️ Do Not Delete This! feature_request_template ⚠️⚠️ --> <!-- Please read our Rules of Conduct: https://opensource.microsoft.com/codeofconduct/ --> <!-- Please search existing issues to avoid creating duplicates. --> This feature request is related to #519, but I would prefer to discuss this new proposal separately (the other issue has hundreds of comments, with many different ideas). We use VS Code at Google and we have an internal issue for this with 314 upvotes. So we decided to take a closer look at the problem and found that it is a request mainly for accessibility, not for more customization. The default guidance is to use the browser zoom to increase the font size. This can work well in some cases, but it leads to less information density (because it makes everything bigger, including margins, etc.). We don't want to let users set their own font size, as it would be hard to test and lead to many UX challenges. Instead, we propose a setting with two or three values. In the mocks below, the default setting is the current font size, "comfortable" increases everything by +1px and "large" makes all UI font-sizes +2px. The sketch is very basic and would need some re-thinking of spacing too. @albertelo made these mocks, with 3 options for a "Font Density" setting Default:  Comfortable:  Large:  If we used the browser zoom instead (as currently recommended), we would have much less information on the last screenshot. Being able to change the size of everything (browser zoom) or just the text (this proposal) is not new. For example, Android accessibility settings have the same options: ![image](https://user-images.githubusercontent.com/485…