Rename vscode-app -> extension-app file to match convention

[charisk]

The ExtensionApp class was located a vscode-app.ts so I've renamed that to be extension-app.ts to match convention.

Checklist

N/A:

• CHANGELOG.md has been updated to incorporate all user visible changes made by this pull request.
• Issues have been created for any UI or other user-facing changes made by this pull request.
• [Maintainers only] If this pull request makes user-facing changes that require documentation changes, open a corresponding docs pull request in the github/codeql repo and add the ready-for-doc-review label there.

[shati-patel]

Makes sense!