Replace rem() function with tokens and postcss plugin

[alex-page]

• Investigate pixrem and pxtorem.

Release steps

• Ship pxtorem in next release
• Remove rem() function from public sass
• Remove px() function from public sass
• Update html font-size to 100% and ensure any rem values are updated accordingly
• Small values like 1px should remain as px units and not be converted to rem by pxtorem #4776

[BPScott]

While you're poking at removing stuff in _utilities.scss, could you remove the em() and px() functions too?

Both the rem and em functions trigger deprecation warnings in dart-sass that can't be resolved without breaking compatibility with node-sass (short of just deleting the functions), so if we're getting rid of one it'd be good to get rid of both.

[aveline]

This issue should also include updating html font-size to 100%