Coding Standards: Replace alias PHP functions with the canonical names.

Using the canonical function name for PHP functions is strongly recommended, as aliases may be deprecated or removed without (much) warning.

This replaces all uses of the following:
* `join()` with `implode()`
* `sizeof()` with `count()`
* `is_writeable()` with `is_writable()`
* `doubleval()` with a `(float)` cast

In part, this is a follow-up to #47746.

Props jrf.
See #50767.

git-svn-id: https://develop.svn.wordpress.org/trunk@49193 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/theme.php

@@ -2149,7 +2149,7 @@ function get_theme_starter_content() {
 				'text',
 				array(
 					'title'  => _x( 'Find Us', 'Theme starter content' ),
-					'text'   => join(
+					'text'   => implode(
 						'',
 						array(
 							'<strong>' . _x( 'Address', 'Theme starter content' ) . "</strong>\n",