meson: Pull in syn from crates.io

We don't have real crates.io support yet so this uses a hack where it
pulls wraps from my personal github.

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24998>

subprojects/packagefiles/unicode-ident/meson.build

@@ -0,0 +1,14 @@
+project(
+  'unicode-ident',
+  'rust',
+  version : '1.0.6',
+  license : '(MIT or Apache-2.0) AND Unicode-DFS-2016)',
+)
+
+lib = static_library(
+  'unicode_ident',
+  'src/lib.rs',
+  override_options : ['rust_std=2018', 'build.rust_std=2018'],
+  rust_abi : 'rust',
+  native : true,
+)