Backport: Bug fixes in theme json with fluid typography and spacing presets

[glendaviesnz]

The new fluid typography setting is missing from the theme.json valid values in 6.1, which causes the value to be dropped from theme exports from the editor as reported here.

Trac ticket: https://core.trac.wordpress.org/ticket/56684

[ramonjd]

Thanks @glendaviesnz

This was an oversight on my part in the main Typgraphy PR that got merged #3237

[ramonjd]

Thank you @glendaviesnz 💟

It's looking good. Should the spacing slug value be a string? E.g., "slug": "30"

2023 theme.json

"typography": {
"dropCap": false,
"fluid": true,
"fontFamilies": [
..."spacing": {
"spacingScale": {
"steps": 0
},
"spacingSizes": [
{
"size": "clamp(1.5rem, 5vw, 2rem)",
"slug": "30",
"name": "30"
},

Export before

"typography": {
"dropCap": false,
"fontFamilies": [
...."spacing": {
"spacingScale": [],
"spacingSizes": [
{
"name": "30",
"size": "clamp(1.5rem, 5vw, 2rem)",
"slug": 30
},

After

"typography": {
"dropCap": false,
"fluid": true,
"fontFamilies": [
..."spacing": {
"spacingScale": {
"steps": 0
},
"spacingSizes": [
{
"name": "30",
"size": "clamp(1.5rem, 5vw, 2rem)",
"slug": 30
},

[ramonjd]

Approving since the spacing slug number will be cast to a string when imported/injested.

The other two changes affect functionality and should be fixed.

[dream-encode]

I was looking to merge this, but I want to make sure this comment by @tellthemachines is addressed in the final PR:
WordPress/gutenberg#44555 (review)

[dream-encode]

After looking at the code and the comments, it appears the concerns are addressed. Prepping a merge commit to core.

[dream-encode]

Merged into core in https://core.trac.wordpress.org/changeset/54360.