Relatively "more bold" discussion

A follow up to #10498.

If the user already has a bold font selected, should we make the font for bold text **even more bold**?

Options:
* when the user has a bold font, leave intense text just **bold**
* increase the font weight **EVEN MORE**, so bold text becomes extra bold (up to a sensible max)
* how does this play when the user's default font weight is something like extra-light? does extra-light -> light -> normal -> bold -> extra-bold?
* possible have a setting to switch between the above behaviors? (might be overkill)
* Allow the user to specify a custom "bold text font weight", so they could have extra-bold text by default, and "bold" text we could render as extra-light (if you're a crazy person) (also overkill)

Relevant comments:

> https://github.com/microsoft/terminal/issues/109#issuecomment-743929268 is probably the comment you're looking for?

> Just multiply by 1.75, cap it at 1000, and call it a day, perhaps. I run font weight 450 locally ("Retina" weight) rather than 400, so this would come out closer to Extra Bold than Bold, which seems reasonable to me. If the font doesn't support variable-width, let DirectWrite work out which weight instance to pick.
>
> If it's helpful, CSS defines [its own transformation for bold](https://developer.mozilla.org/en-US/docs/Web/CSS/font-weight#meaning_of_relative_weights), but that doesn't allow for variable-weight fonts, it relies on their [font rounding calculation](https://developer.mozilla.org/en-US/docs/Web/CSS/font-weight#fallback_weights) to pick a "named" weight first, then enbolden it. I would expect `DirectWrite` to do that for us.

et al in https://github.com/microsoft/terminal/pull/10498/files#r657675585.

Start by reading follow-up issue #10498 and the discussion in pull request #10498, especially the referenced DirectWrite and CSS font-weight considerations. Define which bold-text behavior should be supported, including how custom or variable font weights are handled, then document the agreed result and its limits.

Written by the indexing model from the issue text.