Replace protected consts with consts

[bgrgicak]

Fixes #81

This PR fixes PHP < 7.1 support by removing the use of const visibility modifiers.

Testing instructions

• Checkout this branch
• Install the plugin on a PHP 7.0 or older server

  export WP_ENV_PHP_VERSION="7.0" && wp-env start

• Go to /wp-admin/
• Click on Settings > SQLite integration
• Click _Install SQLite database
• Confirm that there are no PHP errors and the installer starts

[bgrgicak]

@adamziel @brandonpayton this PR should fix Playground crashes on PHP 7.0.

[adamziel]

❤️ Lovely! @aristath would you be able to ship a new release, or give me and @bgrgicak permissions to do that?

[brandonpayton]

@bgrgicak nice!