Migrate the font size classes to use CSS variables

[oandregal]

Follow-up to https://github.com/WordPress/gutenberg/pull/21153/files#r404193920

This PR migrates the font classes to use the CSS variables instead of using the font-size property directly.

[github-actions]

Size Change: +12 B (0%)

Total Size: 889 kB

Filename	Size	Change
build/block-library/style-rtl.css	7.54 kB	+7 B (0%)
build/block-library/style.css	7.55 kB	+5 B (0%)

ℹ️ View Unchanged

Filename	Size	Change
build/a11y/index.js	1.02 kB	0 B
build/annotations/index.js	3.4 kB	0 B
build/api-fetch/index.js	3.79 kB	0 B
build/autop/index.js	2.58 kB	0 B
build/blob/index.js	620 B	0 B
build/block-directory/index.js	6.03 kB	0 B
build/block-directory/style-rtl.css	760 B	0 B
build/block-directory/style.css	760 B	0 B
build/block-editor/index.js	102 kB	0 B
build/block-editor/style-rtl.css	10.2 kB	0 B
build/block-editor/style.css	10.2 kB	0 B
build/block-library/editor-rtl.css	7.22 kB	0 B
build/block-library/editor.css	7.22 kB	0 B
build/block-library/index.js	110 kB	0 B
build/block-library/theme-rtl.css	669 B	0 B
build/block-library/theme.css	671 B	0 B
build/block-serialization-default-parser/index.js	1.65 kB	0 B
build/block-serialization-spec-parser/index.js	3.1 kB	0 B
build/blocks/index.js	57.5 kB	0 B
build/components/index.js	195 kB	0 B
build/components/style-rtl.css	16.6 kB	0 B
build/components/style.css	16.5 kB	0 B
build/compose/index.js	6.21 kB	0 B
build/core-data/index.js	10.7 kB	0 B
build/data-controls/index.js	1.04 kB	0 B
build/data/index.js	8.23 kB	0 B
build/date/index.js	5.36 kB	0 B
build/deprecated/index.js	772 B	0 B
build/dom-ready/index.js	569 B	0 B
build/dom/index.js	3.05 kB	0 B
build/edit-navigation/index.js	2.71 kB	0 B
build/edit-navigation/style-rtl.css	239 B	0 B
build/edit-navigation/style.css	241 B	0 B
build/edit-post/index.js	92.9 kB	0 B
build/edit-post/style-rtl.css	12.3 kB	0 B
build/edit-post/style.css	12.3 kB	0 B
build/edit-site/index.js	10.1 kB	0 B
build/edit-site/style-rtl.css	5.02 kB	0 B
build/edit-site/style.css	5.02 kB	0 B
build/edit-widgets/index.js	7.17 kB	0 B
build/edit-widgets/style-rtl.css	3.74 kB	0 B
build/edit-widgets/style.css	3.73 kB	0 B
build/editor/editor-styles-rtl.css	399 B	0 B
build/editor/editor-styles.css	400 B	0 B
build/editor/index.js	42.8 kB	0 B
build/editor/style-rtl.css	3.49 kB	0 B
build/editor/style.css	3.49 kB	0 B
build/element/index.js	4.44 kB	0 B
build/escape-html/index.js	733 B	0 B
build/format-library/index.js	6.95 kB	0 B
build/format-library/style-rtl.css	502 B	0 B
build/format-library/style.css	502 B	0 B
build/hooks/index.js	1.93 kB	0 B
build/html-entities/index.js	622 B	0 B
build/i18n/index.js	3.57 kB	0 B
build/is-shallow-equal/index.js	710 B	0 B
build/keyboard-shortcuts/index.js	2.3 kB	0 B
build/keycodes/index.js	1.7 kB	0 B
build/list-reusable-blocks/index.js	2.99 kB	0 B
build/list-reusable-blocks/style-rtl.css	226 B	0 B
build/list-reusable-blocks/style.css	226 B	0 B
build/media-utils/index.js	4.84 kB	0 B
build/notices/index.js	1.57 kB	0 B
build/nux/index.js	3.01 kB	0 B
build/nux/style-rtl.css	616 B	0 B
build/nux/style.css	613 B	0 B
build/plugins/index.js	2.54 kB	0 B
build/primitives/index.js	1.5 kB	0 B
build/priority-queue/index.js	789 B	0 B
build/redux-routine/index.js	2.84 kB	0 B
build/rich-text/index.js	14.5 kB	0 B
build/server-side-render/index.js	2.54 kB	0 B
build/shortcode/index.js	1.69 kB	0 B
build/token-list/index.js	1.28 kB	0 B
build/url/index.js	4.01 kB	0 B
build/viewport/index.js	1.6 kB	0 B
build/warning/index.js	1.14 kB	0 B
build/wordcount/index.js	1.17 kB	0 B

_{compressed-size-action}

[oandregal]

In #21428 we're reverting the work on adding CSS variables to blocks by default, so this will have to wait. Going to close in the meantime.