[system] Improve the fontWeight property by improving the values used

[mnajdova]

• I have searched the issues of this repository and believe that this is not a duplicate.

Summary 💡

This is a proposal of how we may improve the fontWeight prop in the system.

This is how it looks currently:

<Box
  fontWeight="fontWeightRegular"
/>

Option 1

The first option of how we may improve it is by the following usage:

<Box
  fontWeight="regular"
/>

where we would pull values from theme.typography.fontWeightX Related to #17615

Option 2

The other option is to use the following:

<Box
  fontWeight="body1.fontWeight"
/>

where we would be still pulling from theme.typography.x.

First reported in #23053 (comment)

I am in favor of the second approach, but I am open for opinion from others :)

[oliviertassinari]

Option 2. is already supported, I think that option 1 and 2 are complementary and will be welcomed.

A proposal, we can make it systematic for the other typography values. We could have:

• x: 'default' pulls from theme.typography.x
• x: 'y' pulls from theme.typography.xY or theme.typography.y.

This would solve #17615 from @lcswillems but also simplify

<Box fontFamily="fontFamily" />

to

<Box fontFamily="default" />