Themes: Improve performance of get_block_theme_folders function

This commit enhances the performance of the get_block_theme_folders function by introducing a new method called get_block_template_folders within the WP_Theme class. Previously, this function suffered from poor performance due to repeated file lookups using file_exists. The new method implements basic caching, storing the result in the theme's cache, similar to how block themes are cached in the block_theme property (see [55236]).

Additionally, this change improves error handling by checking if a theme exists before attempting to look up the file. It also enhances test coverage. 

Props spacedmonkey, thekt12, swissspidy, flixos90, costdev, mukesh27.
Fixes #58319.

git-svn-id: https://develop.svn.wordpress.org/trunk@56621 602fd350-edb4-49c9-b593-d223f7449a82

tests/phpunit/data/themedir1/block-theme-child-deprecated-path/block-template-parts/small-header.html

@@ -0,0 +1,3 @@
+<!-- wp:paragraph -->
+<p>Small Header Template Part</p>
+<!-- /wp:paragraph -->

tests/phpunit/data/themedir1/block-theme-child-deprecated-path/style.css

@@ -0,0 +1,8 @@
+/*
+Theme Name: Block Theme Child Deprecated Path
+Theme URI: https://wordpress.org/
+Description: For testing purposes only.
+Template: block-theme
+Version: 1.0.0
+Text Domain: block-theme-child-deprecated-path
+*/

tests/phpunit/data/themedir1/block-theme-child-deprecated-path/templates/index.html

@@ -0,0 +1,3 @@
+<!-- wp:paragraph -->
+<p>Index Template</p>
+<!-- /wp:paragraph -->

tests/phpunit/data/themedir1/block-theme-child-deprecated-path/theme.json

@@ -0,0 +1,71 @@
+{
+	"version": 1,
+	"settings": {
+		"color": {
+			"palette": [
+				{
+					"slug": "light",
+					"name": "Light",
+					"color": "#f5f7f9"
+				},
+				{
+					"slug": "dark",
+					"name": "Dark",
+					"color": "#000"
+				}
+			],
+			"gradients": [
+				{
+					"name": "Custom gradient",
+					"gradient": "linear-gradient(135deg,rgba(0,0,0) 0%,rgb(0,0,0) 100%)",
+					"slug": "custom-gradient"
+				}
+			],
+			"custom": false,
+			"customGradient": false
+		},
+		"typography": {
+			"fontSizes": [
+				{
+					"name": "Custom",
+					"slug": "custom",
+					"size": "100px"
+				}
+			],
+			"customFontSize": false,
+			"customLineHeight": true
+		},
+		"spacing": {
+			"units": [
+				"rem"
+			],
+			"customPadding": true
+		},
+		"blocks": {
+			"core/paragraph": {
+				"color": {
+					"palette": [
+						{
+							"slug": "light",
+							"name": "Light",
+							"color": "#f5f7f9"
+						}
+					]
+				}
+			}
+		}
+	},
+	"customTemplates": [
+		{
+			"name": "page-home",
+			"title": "Homepage template"
+		}
+	],
+	"templateParts": [
+		{
+			"name": "small-header",
+			"title": "Small Header",
+			"area": "header"
+		}
+	]
+}