www/code-server: Fix extension installation

VS Code's platform detection does not know FreeBSD and falls back to
"Web", so the extension service rejected every non-web extension with
"not available in code-server for the Web platform". Treat FreeBSD as
Linux in platform.ts, the same way editors/vscode does, so extensions
can be installed.

Obtained from: editors/vscode
Sponsored by: Netzkommune GmbH