Update ExtensionsGallery files to ExtensionsMarketplace naming

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 ExtensionsGallery files to ExtensionsMarketplace naming

• This renames all files with "ExtensionsGallery" naming to "ExtensionsMarketplace" naming.
• Variables and methods with "gallery" naming are also updated to "marketplace" naming where appropriate.
• The Web IDE facing extensions gallery references are not updated yet and will be handled in a separate 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.