Add stricter validation for field names

[alexander-schranz]

This outputs a error message for an unexpected input for the field name. We validate currently two things.

• field Key === field name (this may be change in future)
• field name only supports a-z, A-Z, 0-9 and _ (see Identifiers for meilisearch and probably others need to be checked #426)

fix #426

FYI @Toflar

[alexander-schranz]

PS: I'm using internal assert of PHP to avoid overheads in validation for production. For CI using the widely used https://github.com/shivammathur/setup-php It is required to configure zend.assertions=1 via:

            - name: Install and configure PHP
              uses: shivammathur/setup-php@v2
              with:
                  php-version: ${{ matrix.php-version }}
                  tools: 'composer:v2'
                  ini-values: memory_limit=-1, zend.assertions=1

FYI: @Toflar