deps: use fork of fluent-rs on GitHub

This fork differs from mainline `fluent-rs` by a single line, which
makes `Parser` from the `fluent-syntax` crate public. We need this
because our code depends on the parser for much of its functionality.

There is an open PR for this, but so far it hasn't been addressed:
projectfluent/fluent-rs#394

Author: Daniel Rainer

Cargo.lock

@@ -35,8 +35,8 @@ name = "show-missing"
 path = "src/bin/show_missing.rs"
 
 [dependencies]
-fluent = { path = "../fluent-rs/fluent" }
-fluent-syntax = { path = "../fluent-rs/fluent-syntax" }
+fluent = { git = "https://github.com/danielrainer/fluent-rs", rev = "cf712bced280b217b6307edabc2089b3e57204ab" }
+fluent-syntax = { git = "https://github.com/danielrainer/fluent-rs", rev = "cf712bced280b217b6307edabc2089b3e57204ab" }
 nix = { version = "0.30", features = ["fs"] }
 gettext-po-file-parser = { path = "crates/gettext-po-file-parser" }
 unic-langid = "0.9.6"