Global styles: add :placeholder pseudo-element support for textInput

[simison]

Resolves #71382

Adds support for styling :placeholder in textInput elements (added in #70378)

What?

Closes

Why?

Currently, the :placeholder pseudo element cannot be set via theme.json because it is not included in the VALID_ELEMENT_PSEUDO_SELECTORS list. This limitation makes it difficult to implement consistent styles for form inputs in plugins across the theme, and themers need to rely on custom CSS.

Later on, it would be good to add UI for allowing modification of the style directly in the editor.

How?

The PR updates the VALID_ELEMENT_PSEUDO_SELECTORS array in class-wp-theme-json-gutenberg.php by adding :placeholder. This change allows the :placeholder pseudo-class to be used directly within theme.json files.

Testing Instructions

1. Apply this PR.
2. Create or edit a theme that uses theme.json.
3. Add a style using the :focus-visible pseudo-class in theme.json. For example:

{
	"styles": {
		"elements": {
			"textInput": {
				":placeholder": {
					"color": {
						"text": "red"
					}
				}
			},
		}
	}
}

Testing Instructions for Keyboard

Screenshots or screencast

Before	After

[simison]

There's some work already in #71634 so I'm closing this one :-)