Update font weights

[xak]

The 72 font does not include medium or semi weights so we should not support those values.

The weights in the main map should match those available with the fonts.

Update

• Remove unsupported weights from the settings map

$fd-weights: map-merge((
    light: 300,
    normal: 400, //see note below
    bold: 700
), $fd-weights);

• Update weight mixin to accept only the 3 weights supported.

This should be completed re-written to look up the weight in the map. It does not do that now.

Look up the weight passed to the mixin, if the value is invalid then return normal.

• Change all references of reg to normal.
  Existing mixins allow for reg to be passed to return the normal or 400 weight. Regular is more of a typographic term and not a CSS term.

Recommend maintaining the reg param for now but docs should be updated to use normal

@mixin fd-weight($weight: normal) {
    @if $weight == reg {
        $weight: normal
    }
    ...
   }
}

[xak]

This is done as part of #741