refactor(src) is_file over file_exists

Replace uses of `file_exists` with `is_file`. While there is
functionally no difference between the two in this context, the
`is_file` function will use a cached check for a small performance
boost.

src/Assets/Asset.php

@@ -622,13 +622,13 @@ protected function build_min_asset_url( $original_url ): string {
 
 		$script_debug = defined( 'SCRIPT_DEBUG' ) && Utils::is_truthy( SCRIPT_DEBUG );
 
-		if ( $script_debug && file_exists( wp_normalize_path( $root_path . $resource_path . $resource ) ) ) {
+		if ( $script_debug && is_file( wp_normalize_path( $root_path . $resource_path . $resource ) ) ) {
 			return $original_url;
 		}
 
 		$minified_abs_file_path = wp_normalize_path( $root_path . $minified_file_path );
 
-		if ( ! file_exists( $minified_abs_file_path ) ) {
+		if ( ! is_file( $minified_abs_file_path ) ) {
 			return $original_url;
 		}
 
@@ -1140,7 +1140,7 @@ public function has_asset_file(): bool {
 			return false;
 		}
 
-		return file_exists( $asset_file_path );
+		return is_file( $asset_file_path );
 	}
 
 	/**
@@ -1381,7 +1381,7 @@ public function maybe_get_min_file( $url ) {
 			$file_path = wp_normalize_path( "{$base_dir}/{$partial_path}" );
 			$file_url  = "{$base_url}/{$partial_path}";
 
-			if ( file_exists( $file_path ) ) {
+			if ( is_file( $file_path ) ) {
 				return $file_url;
 			}
 		}

tests/wpunit/AssetsTest.php

@@ -971,7 +971,7 @@ protected function assert_minified_found( $slug_prefix, $is_js = true, $has_min
 		$plugins_path = str_replace( constant( 'WP_CONTENT_DIR' ), '', constant( 'WP_PLUGIN_DIR' ) );
 
 		if ( constant( 'WP_PLUGIN_DIR' ) !== constant( 'WP_CONTENT_DIR' ) . $plugins_path || strpos( constant( 'ABSPATH' ), 'C:') === 0 || $wont_figure_out_min_vs_unmin ) {
-			// If we are testing outside of the actual plugin directory, the file_exists will always fail.
+			// If we are testing outside of the actual plugin directory, the `is_file` check will always fail.
 			// In installations where this set up is the actual, the file should exist.
 			// In this case it will always fail to locate mins.
 			$urls = array_map(