Update composer packages

[kenjis]

We can't run composer install with Composer v2.
This PR fixes it.

$ composer install
Installing dependencies from lock file (including require-dev)
Verifying lock file contents can be installed on current platform.
Your lock file does not contain a compatible set of packages. Please run composer update.

  Problem 1
    - dealerdirect/phpcodesniffer-composer-installer is locked to version v0.5.0 and an update of this package was not requested.
    - dealerdirect/phpcodesniffer-composer-installer v0.5.0 requires composer-plugin-api ^1.0 -> found composer-plugin-api[2.0.0] but it does not match the constraint.

You are using Composer 2, which some of your plugins seem to be incompatible with. Make sure you update your plugins or report a plugin-issue to ask them to support Composer 2.

Includes:

• update phpcodesniffer-composer-installer for composer v2
• update PHPUnit to ^7.0
• update Travis PHP versions
• fix coding style with vendor/squizlabs/php_codesniffer/phpcs.xml.dist
• remove composer.lock
• add composer test command

[kenjis]

$ vendor/bin/phpunit --filter CodeIgniter4
PHPUnit 6.5.14 by Sebastian Bergmann and contributors.

.........                                                           9 / 9 (100%)

9 sniff test files generated 25 unique error codes; 22 were fixable (88%)

Time: 3.51 seconds, Memory: 30.00MB

OK (9 tests, 0 assertions)

Generating code coverage report in Clover XML format ... done

Generating code coverage report in HTML format ... done

[kenjis]

It seems we must update PHPUnit to version 9.3.

https://travis-ci.org/github/codeigniter4/coding-standard/jobs/761060422

Your requirements could not be resolved to an installable set of packages.

  Problem 1

    - phpunit/phpunit[6.5.0, ..., 6.5.14] require php ^7.0 -> your php version (8.0.2) does not satisfy that requirement.

    - Root composer.json requires phpunit/phpunit ^6.5 -> satisfiable by phpunit/phpunit[6.5.0, ..., 6.5.14].

[kenjis]

PHP_CodeSniffer does not support PHPUnit 8 or later:
https://github.com/squizlabs/PHP_CodeSniffer/blob/1106d65547e581a5184a9293486d9e6d81595462/composer.json#L34

[kenjis]

@louisl Could you review this?

[louisl]

@kenjis Thanks for the PR, I'll check it out ASAP.

[kenjis]

@louisl Thanks!

[kenjis]

@louisl When I composer update in CI4, codeigniter4-standard does not update.
See #37