Release v18.0.0 · primer/css

#1599 13de16c7 Thanks @jonrohan! - Removing the rem() mixin and usages. Placing the computed values in place.
#1615 612841f1 Thanks @simurai! - Update v1 deprecations
#1599 13de16c7 Thanks @jonrohan! - Rename modes.scss file to color-modes.scss and move from the base/ folder to the support/variables folder.
#1599 13de16c7 Thanks @jonrohan! - Moving color modes to their own bundle, ./color-modes/ and separates color mode themes into their own scss file.
#1602 852da9d5 Thanks @simurai! - Remove V1 color utilities
#1617 e47324fa Thanks @jonrohan! - Removing <kbd> import from markdown package. Going forward you'll need to include @primer/css/base/kbd.scss directly.
#1630 5f7bb45c Thanks @simurai! - Update utilities
#1599 13de16c7 Thanks @jonrohan! - Remove break-word from utilities

#1601 410c1d63 Thanks @jonrohan! - Add light & dark colorblind themes
#1626 4d00d51d Thanks @jonrohan! - Changing to @primer/stylelint-config and upgrading @primer/primitives to 5.0 rc
#1616 afac04bc Thanks @talum! - Adding footnote styles to markdown-body.
#1617 e47324fa Thanks @jonrohan! - Convert postcss build tool, from node-sass to dart-sass.
#1603 21c5ebc0 Thanks @simurai! - Update docs with V2 colors

#1628 ce197c9f Thanks @edokoa! - Update flash alert icons
#1629 54d3068b Thanks @jdanyow! - remove remaining color-border-primary refs
#1631 5f3689b9 Thanks @jonrohan! - Updating @primer/stylelint-config and @primer/primitives to 5.0
#1659 d32e42f9 Thanks @jonrohan! - Rename protanopia themes to colorblind.
#1658 0ed0307b Thanks @koddsson! - Increase the z-index value for the details dialog overlay.
#1661 ebb573ff Thanks @simurai! - Update bg deprecations

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