FVM_DIR constant can’t be used to change cache location

Hi Raul,

First of all, awesome plugin you have here but it seems overriding the cache location to the common dynamic writable path of WordPress in wp-content/uploads via FVM_DIR is not working. That override is useful especially on a version-controlled codebase but the documented override by the plugin does not work when using the FVM_DIR constant as suggested by the doc https://github.com/peixotorms/fast-velocity-minify/blob/f2dc31229eac0f68f7ce49a6a2927d9fc656b59d/readme.txt#L35 because FVM_DIR is used in the plugin
to also reference the plugin path https://github.com/peixotorms/fast-velocity-minify/blob/f2dc31229eac0f68f7ce49a6a2927d9fc656b59d/fvm.php#L38-L44 resulting to a fatal error when enabling the plugin:

example when using define( 'FVM_DIR', 'wp-content/uploads/' );

Fatal error: require_once(): Failed opening required 'wp-content/uploads/inc/common.php' (include_path='.:/usr/share/pear:/usr/share/php') in wp-content/plugins/fast-velocity-minify/fvm.php on line 47

Steps to reproduce:

1. Add in the wp-config.php

define( 'FVM_DIR', '/code/wp-content/uploads/' );
define( 'FVM_URL', WP_SITEURL . '/wp-content/uploads/' );

2. By enabling the plugin, you will get the error above

Proposal:
Add in the function fvm_get_cache_location() a specific CONSTANT that references the specific cache path but is also not used to reference the plugin path https://github.com/peixotorms/fast-velocity-minify/blob/f2dc31229eac0f68f7ce49a6a2927d9fc656b59d/fvm.php#L38-L44 example using FVM_CACHE_DIR & FVM_CACHE_URL

	if (defined('FVM_CACHE_DIR') && defined('FVM_CACHE_URL')){
	
		# define paths and url
		$sep = DIRECTORY_SEPARATOR;
		$dir = trim(rtrim(FVM_CACHE_DIR, '/\\')). $sep . 'cache' . $sep . 'fvm'. $sep . 'min';
		$durl = trim(rtrim(FVM_CACHE_URL, '/')). '/cache/fvm/min';
		
		# create and return
		if(!is_dir($dir) && function_exists('wp_mkdir_p')) { wp_mkdir_p($dir); }
		return array('ch_dir'=>$dir,'ch_url'=>$durl);
		
	}

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