Update extensions_gallery internal setting names to marketplace

Issue: Standardize extensions marketplace terminology (#512642 - closed)

• Part 1: Update extensions_gallery internal setting name... (!180981 - merged)
• Part 2: Update ExtensionsGallery files to ExtensionsMar... (!180988 - merged)
• Part 3: Rename extensionsGallery names to extensionMark... (!181010 - merged)

What does this MR do and why?

Update extensions_gallery internal setting names to marketplace

• This supports this issue to standardize our internal naming to the name of the feature.
• This only focuses on the vscode_extension_marketplace and vscode_extension_marketplace_metadata setting names, as well as the vscode_extension_marketplace_feature_flag option.
• Other renamings will happen in future commits.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

No changes. Everything still works as expected 👍

Extension marketplace disabled	Extension marketplace enabled

How to set up and validate locally

1. Enable web_ide_extensions_marketplace feature flag.
2. Visit user preferences and enable the extension marketplace under Web IDE and Workspaces section.
3. Visit Web IDE for any project.
4. The Web IDE should start up normally and extension marketplace functional.