Refactor virtual code URI format

[johnsoncodehk]

Since 2.0, the format of the URI of virtual code's document has changed to:

{originalUri}?virtualCodeId={virtualCodeId}

Which seems to be semantically inaccurate. We can consider changing to the format used by VSCode's embedded language programming example:

volar-embedded-content://{encodeURIComponent(virtualCodeId)}/{encodeURIComponent(originalUri)}