Convert class to singlton

Suggestion so we can furthre hook into your plugin for when using other custom WordPress authentication hooks.

add a static function instance() in order to initiate the class via this static file (as singleton instance)

#!php
<?php

public static $instance;
public static function instance($params = []) {
    if (!self::$instance) self::$instance = new self($params);
    return(self::$instance);
}

at the end of the file initiate the instance

#!php
<?php
SimpleGoogleRecaptcha::instance();

instead of the
new SimpleGoogleRecaptcha();

this way we can hook into with the recaptcha using this plugin, as simple as similar to this ‘partial code’

#!php
<?php
add_filter( 'wp_authenticate_user', function( $user, $password ) {
    if ( !class_exists( 'SimpleGoogleRecaptcha' ) ) return( $user );
    $recaptcha = SimpleGoogleRecaptcha::instance();
    if ( !$recaptcha->verify( $user ) ) {
        return new WP_Error(
            'invalid_recaptcha', // incorrect_password
            __( 'Recaptcha Invalid, Check and try again.' )
        );
    }
}, 50, 2 );