feat(brand): Allow named weights and numeric values in brand-font-weight

[gadenbuie]

This PR updates the brand-font-weight definition to allow named font weight values and to support variable font weights. Currently the definition limits font weight to 1:9 * 100:

- id: brand-font-weight
  description: A font weight.
  enum: [100, 200, 300, 400, 500, 600, 700, 800, 900]
  default: 400

but CSS also allows normal (400), bold (700), lighter, and bolder. In CSS, the last two are relative font weights and are relative to the element's inherited value. In our case, however, I'd recommend we simply assign these values 100 (lighter) and 900 (bolder).

Here's the description of font-weight as a numeric value from MDN:

<number>
A <number> value between 1 and 1000, both values included. Higher numbers represent weights that are bolder than (or as bold as) lower numbers. This allows fine-grain control for variable fonts. For non-variable fonts, if the exact specified weight is unavailable, a fallback weight algorithm is used — numeric values that are divisible by 100 correspond to common weight names, as described in the Common weight name mapping section below.

In HTML outputs, we can pass numeric and named values through and let CSS handle the computation. In places where variable or named font weights aren't supported, I propose we round to the nearest named point (using our definition for lighter and bolder):

Name	Value
lighter	100
normal	400
bold	700
bolder	900

[gadenbuie]

Instead of lighter and bolder, let's use absolute terms from common weight names.

[cscheid]

Quarto will add:

  - thin
  - extra-light
  - light
  - normal
  - regular
  - medium
  - semi-bold
  - demi-bold
  - bold
  - extra-bold
  - ultra-bold
  - black

[cscheid]

I'm going to close this in favor of 97b5660

[gadenbuie]

Looks great, thanks!