feat: export the used crypto runtime as a constant

This will aid libraries to determine which jose runtime was
imported/required.

With the many JS runtimes each having a different way of resolving
transient dependencies and targets this has become a bit troublesome
to deal with.

src/index.ts

@@ -96,3 +96,5 @@ export type {
   JWTHeaderParameters,
   JSONWebKeySet,
 } from './types.d'
+
+export { default as cryptoRuntime } from './runtime/runtime.js'

src/runtime/browser/runtime.ts

@@ -0,0 +1,3 @@
+import { CryptoRuntime } from '../../types.d'
+
+export default 'WebCryptoAPI' as CryptoRuntime

src/runtime/node/runtime.ts

@@ -0,0 +1,3 @@
+import { CryptoRuntime } from '../../types.d'
+
+export default 'node:crypto' as CryptoRuntime

src/types.d.ts

@@ -668,3 +668,5 @@ export interface CompactJWEHeaderParameters extends JWEHeaderParameters {
 export interface JSONWebKeySet {
   keys: JWK[]
 }
+
+export type CryptoRuntime = 'WebCryptoAPI' | 'node:crypto'