Add PHP CodeSniffer and fix reported linting errors (#18)

Adds PHP CodeSniffer to lint the codebase. Also auto fixes errors related to formatting. This change allows all developers to follow the same coding patterns and agree on new ones by changing the common ruleset.

- Uses composer to install code sniffer and required rules
- Add composer to Docker image
- Add phpcs rules for linting
- Update the contribution guide with information about linting the code

.gitignore

@@ -1 +1,2 @@
 logs/
+vendor/

Dockerfile

@@ -1,5 +1,13 @@
 FROM wordpress:6.6.2
 
+# Install Composer.
+RUN cd /tmp \
+    && php -r "copy('https://getcomposer.org/installer', 'composer-setup.php');" \
+    && php -r "if (hash_file('sha384', 'composer-setup.php') === 'dac665fdc30fdd8ec78b38b9800061b4150413ff2e3b6f88543c636f7cd84f6db9189d43a81e5503cda447da73c7e5b6') { echo 'Installer verified'; } else { echo 'Installer corrupt'; unlink('composer-setup.php'); } echo PHP_EOL;" \
+    && php composer-setup.php \
+    && php -r "unlink('composer-setup.php');" \
+    && mv composer.phar /usr/local/bin/composer
+
 # Install required packages.
 RUN apt-get update \
     && apt-get install -y \