Em spacers

[broccolini]

This pr adds em spacer variables to primer-support. The scale is based on commonly used fractions that work with our typography and line-height scale.

Fixes: #397

This is already mostly documented in the styleguide, have updated to include the variables:

Variable	Fraction	Y Padding (em)	Total height at 14px	Total height at 16px
$em-spacer-1	1/16	.0625	22.75	26
$em-spacer-2	1/8	.125	24.5	28
$em-spacer-3	1/4	.25	28	32
$em-spacer-4	3/8	.375	31.5	36
$em-spacer-5	1/2	.5	35	40
$em-spacer-6	3/4	.75	42	48

To do:

• update docs
• update primer components to use spacers
• test in github/github

/cc @primer/design-systems

[broccolini]

Review-lab branch for testing in github here: https://primer-em-spacers.review-lab.github.com