Code cleanup and updated phpunit to support php 7.8

Resolve #2
Updated phpunit/phpunit from 6.4 to 9.5.1

Contributors: @thibaultjunin

Updated es locale

fix: update es locales

thx for the translator

fix require rule and add notNull rule

[BREAKING CHANGES]

fix: the require rule will accept null values
feat: add notNull rule

before this version, a field that was present but with a null value caught the attention of the 'required' rule.
Now with this fix the 'required' rule will only check if a certain key is present in the params or payload.

change requested by @thibaultjunin

Add nested key support

no breaking changes

1.2

BREAKING CHANGES

removal of withKeys() method

1.1.3

added requiredAndNotEmpty alias method requested by @thibaultjunin

hope you enjoy it!

1.1.2

#1 Add alpha numerical and pattern match validation rules

1.1.1

adding exists() helper method on main class

1.1.0

feat: add i18n docs in README.md

1.0.9

[experimental feature]: add boolean validation