WP_Theme_JSON: Prevent implicit coercion in to_ruleset

`to_ruleset` used string concatenation (`$element['name'] . ': ' . $element['value'] . ';'`), so PHP implicitly coerced non-string values (e.g. booleans → `'1'`/`''`, arrays → `'Array'`). That could emit invalid or misleading CSS.

At the same time, pass a `style` theme.json path in `test_get_styles_with_appearance_tools()` to simulate a style node. Before it was `settings`.

Props ramonopoly, andrewserong, isabel_brison.

Fixes #64848.

 --This line, and those below, will be ignored--

M    src/wp-includes/class-wp-theme-json.php
M    tests/phpunit/tests/theme/wpThemeJson.php


git-svn-id: https://develop.svn.wordpress.org/trunk@62347 602fd350-edb4-49c9-b593-d223f7449a82