R1:3984c14260d3

Tokenize external editor links so they can be safely materialized on the client

Summary:
Ref T13515. Currently, opening a file to a particular line in an external editor relies on replacing "%l" with "%l" (which is escaped as "%25l") on the server, and then replacing "%25l" with the line number on the client. This will fail if the file path (or any other variable) contains "%l" in its unencoded form.

The parser also can't identify invalid variables.

Pull the parser out, formalize it, and make it…