feat: public config helpers for third-party integrations (#25)

Adds invisible_playwright.config module with:
- get_default_stealth_prefs(seed, *, pin, locale, timezone,
  extra_prefs, humanize, virtual_display) -> dict
- get_default_args() -> list

Both also re-exported at the package root alongside the existing
InvisiblePlaywright. ensure_binary is also re-exported there for
parity with the cloakbrowser.download.ensure_binary integration
pattern that downstream projects (Skyvern PR #5340, crawlee-python
PR #1794, agno PR #8129) already expect.

These helpers let third-party fetchers (changedetection.io plugins,
Crawlee BrowserPool subclasses, agno toolkits) drive
playwright.firefox.launch(executable_path=..., firefox_user_prefs=...)
themselves without depending on the InvisiblePlaywright context
manager owning the lifecycle. Same seed semantics, same humanize
toggle, same extra_prefs overlay as the existing wrapper.

Tests: tests/unit/test_config_public.py adds 14 unit tests covering
deterministic seed, locale/timezone/pin/extra_prefs/humanize
variations, and round-trip via the public namespace. Full unit suite
(392 tests) stays green.

Backwards compatible: InvisiblePlaywright surface is unchanged.
BINARY_VERSION stays at firefox-7. Python-only release.

pyproject.toml

@@ -4,7 +4,7 @@ build-backend = "hatchling.build"
 
 [project]
 name = "invisible-playwright"
-version = "0.1.9"
+version = "0.2.0"
 description = "Playwright wrapper for a patched Firefox with deterministic stealth profile."
 readme = "README.md"
 requires-python = ">=3.11"