Add ddev commands for phpstan, phpcs and cspell

[amateescu]

• ddev phpstan [paths]: uses core/phpstan.neon.dist for full runs and core/phpstan-partial.neon when specific paths are given, matching what commit-code-check.sh does (the baseline only applies cleanly to whole-codebase runs). Disables APC like core's script.
• ddev phpcs [paths]: runs with core/phpcs.xml.dist, -ps and --parallel=$(nproc), mirroring core's invocation.
• ddev cspell [globs]: mirrors core's spellcheck:core yarn script (project-root-relative globs, shared cache); checks the same default file set when run without arguments, and prints install instructions when core/node_modules is missing.
• Documents all three in the README and DRUPAL-DEV.md, and extends the bats health checks (positive runs for phpstan/phpcs, the not-installed error path for cspell).