Use ex unit for measure values

[thisisdano]

Preview

---

This updates our measure tokens to use ex units instead of ch per the reasoning in #2960.

This rebalances our measure tokens to be concentrated within our guidance range of 45-90 characters (see https://practicaltypography.com/line-length.html) and should result in output that's more consistent from typeface to typeface because the ex is the height of the x character and our normalization scheme tends to make this value more consistent from face to face.

Now our docs and usa-prose measure are what they say they are: ~75 characters (measure 4).

However, this is something of a breaking change in that the new measure tokens tend to be narrower than their old values and will need to be updated per the chart that follows.

token	desired output	new value	old value	old output in Public Sans
1	~50 characters	44ex	40ch	51 characters
2	~66 characters	60ex	60ch	80 characters
3	~72 characters	64ex	66ch	88 characters
4	~78 characters	68ex	72ch	95 characters
5	~86 characters	74ex	77ch	100 characters
6	~96 characters	88ex	—	—

Beta 6	Beta 7
'1'	'1'
'2'	'4'
'3'	'5'
'4'	'6'
'5'	'6'

---

Fixes #2960

[thisisdano]

Hm, whuts this new Snyk thing? 🤔

[saracope]

Snyk checks every PR for new vulnerabilities.
I just updated for these specific vulnerabilities. If you pull the latest version of 2.0 it should resolve this.

[maya]

LGTM. We'll also need to update the docs to reflect these changes.

[thisisdano]

Yeah, I should have a docs update by later today as part of a big docs/typesetting update