This is a slimmer version of #14335, which only includes the default line heights & the <small> fix.

In this PR:

• Fixed line heights are replaced by a default line height css function.
• This removes the need of adding fixed line heights to each font size.
• It's still possible to use fixed line heights the way it worked before.
• Custom font sizes will now have a better line height by default. For example, text-[2.4rem] will have a calculated line height of 2.625rem, instead of 3.6rem.
• The default values are calculated in CSS, which means you can use arbitrary values in combination with custom properties (eg text-[length:var(--title-font-size)]).
• Relative font sizes like text-[1.5em] or text-[larger] will now use a recalculated line height.
• Only breaking change is the line height of lg (also see Change line height of the lg font size to 1.625rem #14223).
• The line heights are always rounded to even pixels (2px), so that you never have vertical alignment issues. Even if you use viewport relative units.

Here's have an overview of which line height will be used for each font size range:

_{* These font sizes are not used in Tailwind by default}

Closes #14484, #14223