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 generate an intermediate representation which can be sent to the client and reconstituted.

(This temporarily breaks Diffusion and permanently removes the weird, ancient integration in Dark Console.)

Test Plan:

• Added a bunch of tests for the actual parser.
• Used "Open in Editor" in Differential.

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21143