feat: support system-installed browsers via channel and executable path

[SantosVilanculos]

Summary

Adds two new pest()->browser() configuration options to launch system-installed browsers instead of the Playwright-bundled browsers:

// Google Chrome / Microsoft Edge (Playwright's "channel" mechanism)
pest()->browser()->usingChannel('chrome');

// any browser binary, e.g. Firefox
pest()->browser()->usingExecutablePath('/usr/bin/firefox');

Motivation

No downloaded Playwright browser bundles are needed — great for environments that already have Chrome/Firefox/Edge installed and want to avoid npx playwright install.

Implementation

• Configuration::usingChannel(string) and Configuration::usingExecutablePath(string) config methods
• Static channel/executablePath state on Playwright
• Both forwarded as launch options in the Playwright client
• Playwright's run-server is started with --unsafe when an executablePath is set, because the server otherwise filters that launch option out (see filterLaunchOptions in playwright-core); channel is always accepted

Tests

• New tests/Unit/Configuration/SystemBrowserConfigurationTest.php
• All tests/Unit/Configuration + tests/Unit/Playwright pass, Pint + arch pass

Notes

• channel is the recommended approach for Chromium-based system browsers and requires no server flags.
• executablePath requires the --unsafe server mode (added conditionally).