Add preference for release channel

[timtmok]

Address #6489

Adds a preference for choosing where to get updates. The environment variable can still override the preference.

This allows users to switch to getting updates from the dailies.

Altering the preference will prompt the user to restart for the changes to take effect.

Release Notes

New Features

• Add release channel preference Support updating to daily builds #6489

Bug Fixes

• N/A

QA Notes

[Copilot]

Pull Request Overview

This PR adds a configurable release channel preference for Positron updates.

• Introduces a new nested configuration property "update.positron.channel" to allow selection of the update channel.
• Updates the relauncher and update service to observe and use the new "positron.channel" configuration.
• Removes the old UpdateChannel enum and adds a corresponding configuration contribution with limited enum options.

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
src/vs/workbench/contrib/relauncher/browser/relauncher.contribution.ts	Added a "positron" object with a "channel" property to the update configuration and registered it to trigger relaunch on change.
src/vs/platform/update/electron-main/abstractUpdateService.ts	Replaced the deprecated UpdateChannel enum with a dynamic configuration value for the update channel.
src/vs/platform/update/common/update.config.contribution.ts	Introduced a new configuration entry for "update.positron.channel" with specific allowed values and descriptions.

[github-actions]

E2E Tests 🚀
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