Rename extensionsGallery names to extensionMarketplace

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?

Rename extensionsGallery names to extensionMarketplace

• This finalizes renaming variables and methods using "Extensions Gallery" naming to "Extension Marketplace".
• The Web IDE setting extensionsGallerySetting will be updated at a later time when the Web IDE is updated and deployed to support the new name (see MR).

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 visual changes. Everything should still work as expected 🎉

Extensions marketplace disabled	Extensions 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.
3. Visit Web IDE for any project.
4. The Web IDE should start up normally and extension marketplace functional.