js: codegen fixes, W.I.P os availability for JS backend (#11281)

vlib/os/environment.js.v

@@ -0,0 +1,37 @@
+module os
+
+$if js_node {
+	#const $ENV = $process.env
+} $else {
+	#const $ENV = {}
+}
+
+// setenv sets the value of an environment variable with `name` to `value`.
+pub fn setenv(key string, val string, overwrite bool) {
+	#if ($ENV[key] && !(overwrite.valueOf())) return;
+	#$ENV[key] = val + '';
+}
+
+// `getenv` returns the value of the environment variable named by the key.
+pub fn getenv(key string) string {
+	mut res := ''
+	#if ($ENV[key]) res = new builtin.string($ENV[key])
+
+	return res
+}
+
+// unsetenv clears an environment variable with `name`.
+pub fn unsetenv(name string) int {
+	#$ENV[name] = ""
+
+	return 1
+}
+
+pub fn environ() map[string]string {
+	mut res := map[string]string{}
+	#for (const key in $ENV) {
+	#res.map.set(key,$ENV[key])
+	#}
+
+	return res
+}