fix: Enabling developer mode via chrome://extensions

[PatrykKuniczak]

Problem

extensions.ui.developer_mode set via --chromium-pref has no effect, because Chrome
protects it with a MAC in Secure Preferences, so a value written to the
profile before startup is discarded. Verified on macOS and Windows.

Solution

After Extensions.loadUnpacked, if extensions.ui.developer_mode is true
(the default), open chrome://extensions over the CDP pipe, call
chrome.developerPrivate.updateProfileConfiguration({inDeveloperMode: true})
and close the tab. Failures only log a warning. Setting the pref to false
still disables this.

The --chromium-pref help now notes that other protected prefs (e.g.
homepage, session.startup_urls) cannot be set this way That's uncheck by me, suggested by AI).

Tests

Unit tests with a mocked cdp.sendCommand. Manually tested on Chrome 155
(macOS and Windows).

Alternatives

I think it's good idea to create new flag --chromium-secure-pref, but it's out of scope for this PR.
And i'm not really into chrome and chrome extensions world and i don't know what is secure props for real and if this new flag is really necessary.

With this PR i only want to fix my mistake from #3664 and make it finally working.
Because we've finally add it to our project(wxt-dev/wxt#2663), but sadly it doesn't work 😢

I hope this can be merged fast, and we'll be finally able to use it during development 😄

Thanks for your work guys, you've been fueling our project.

[PatrykKuniczak]

@Rob--W You've approved my PR, let's clean our mess together.
That time permanently(I hope, or even for a period till chrome change sth again 😆 )