Walk up directory tree to find php-cs-fixer config

[benmatselby]

Resolves #226

Summary

The Fixer command previously set its working directory to the target file's immediate parent directory. This meant php-cs-fixer could miss project-level config files (e.g. .php-cs-fixer.php) when editing files in nested subdirectories. This PR adds a find_config_dir method that walks up the directory tree from the target file looking for a recognised php-cs-fixer config file, and uses that directory as the working directory. If no config file is found, it falls back to the previous behaviour.

Changes

• phpcs.py: Added CONFIG_FILES class constant listing the four config file names php-cs-fixer recognises (.php-cs-fixer.php, .php-cs-fixer.dist.php, .php_cs, .php_cs.dist)
• phpcs.py: Added Fixer.find_config_dir() method that walks up from a starting directory to the filesystem root, checking for any of the config files at each level
• phpcs.py: Updated Fixer.execute() to use find_config_dir() to determine the working directory instead of always using the target file's parent directory
• tests/test_phpcs.py: Updated existing test_fixer_sets_working_dir test to mock os.path.isfile (renamed to test_fixer_sets_working_dir_to_file_dir_when_no_config)
• tests/test_phpcs.py: Added test_fixer_sets_working_dir_to_config_dir integration-style test
• tests/test_phpcs.py: Added TestFixerFindConfigDir test class with 6 tests covering: no config found, config in start dir, config in parent dir, dist config, legacy config, and priority ordering

[benmatselby]

Might come back to this at a later date