fix(rescript-openapi): use license-file for crates.io compatibility

crates.io rejects packages with non-SPDX license identifiers.
PMPL-1.0-or-later is not yet in the SPDX registry, so switch from
`license = "PMPL-1.0-or-later"` to `license-file = "LICENSE"` which
points to the full PMPL license text. This is the standard workaround
for custom/pending-registration licenses on crates.io.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

rescript-ecosystem/packages/bindings/openapi/Cargo.toml

@@ -8,7 +8,7 @@ edition = "2021"
 authors = ["Jonathan D.A. Jewell <j.d.a.jewell@open.ac.uk>"]
 description = "Generate type-safe ReScript clients from OpenAPI specifications"
 readme = "README.adoc"
-license = "PMPL-1.0-or-later"
+license-file = "LICENSE"
 repository = "https://github.com/hyperpolymath/rescript-openapi"
 keywords = ["rescript", "openapi", "codegen", "api", "client"]
 categories = ["development-tools", "web-programming"]