feat: Add --disable-playwright flag to prevent Playwright installation and prompts

[ei-grad]

This PR introduces a new command-line argument --disable-playwright to address user requests for a way to prevent Aider from prompting for Playwright installation or attempting to use it for web scraping.

Problem:
Users, particularly those running Aider in automated environments or on operating systems not supported by Playwright, found the repeated prompts to install Playwright disruptive (as reported in #3765). Even with --yes-always, the installation process could hang waiting for sudo passwords.

Solution:

• Added a --disable-playwright flag (defaulting to False).
• When --disable-playwright is set to True, Aider will:
  • Skip the check/prompt for Playwright installation within the /web command.
  • Initialize the Scraper with playwright_available=False, ensuring it falls back to using httpx for scraping.
• Added tests to verify the behavior of the flag and ensure no warnings are emitted when Playwright is explicitly disabled.

This change allows users to completely opt-out of Playwright functionality if desired.

Fixes #3765