Fix reading env vars for JS and WASM JS (only NodeJS) (#335)

While working on
#332, I found out
that the implementations for JS and WASM JS are missing. This PR focuses
on adding them. I don't want to add these in the PR that is related
solely to kotlinizing and commonizing the tests to give this fix a
proper visibility in the release notes.

This change was tested within
#332, and the goal
is to merge that PR soon after this one lands.

Note: env vars work only in a non-browser environment.

src/jsMain/kotlin/it/krzeminski/snakeyaml/engine/kmp/internal/system.js.kt

@@ -1,5 +1,14 @@
 package it.krzeminski.snakeyaml.engine.kmp.internal
 
+internal actual fun getEnvironmentVariable(key: String): String? {
+    return process.env[key] as String?
+}
+
+private external val process: Process
+
+private external interface Process {
+    val env: dynamic
+}
 
 //region Identity hash code
 // https://github.com/korlibs/korge/blob/60af53460dd2d68b6ac86cf459d82434e74be629/kds/src/jsMain/kotlin/korlibs/datastructure/internal/InternalJs.kt

src/wasmJsMain/kotlin/it/krzeminski/snakeyaml/engine/kmp/internal/system.wasm.kt

@@ -1,5 +1,9 @@
 package it.krzeminski.snakeyaml.engine.kmp.internal
 
+internal actual fun getEnvironmentVariable(key: String): String? {
+    js("return process.env[key]")
+}
+
 //region Identity hash code
 internal actual fun objectIdentityHashCode(any: Any): IdentityHashCode {
     val ref = any.toJsReference()