Add bring-your-own-buffer versions of os.lookup_env and os.get_env

And make `core:terminal` use it so that `core:log` can be imported with `-default-to-nil-allocator`,
in which the actual allocator is set up in `main()`.

Windows was tricky because of the utf-8 <> utf-16 conversion, so we use some temporary stack buffers for that purpose,
limiting the non-allocating version there to 512 utf-16 characters each for the key and environment value.

In general the value is (obviously) limited to the size of the supplied buffer, and a `.Buffer_Full` error is returned
if that buffer is insufficient. If the key is not found, the procedure returns `.Env_Var_Not_Found`.

TODO:
- Factor out buffer-backed utf8 + utf16 conversion to `core:sys/util` to more easily apply this pattern.
- Add similar `lookup_env` and `get_env` procedures to `core:os/os2`.

Fixes #5336

core/os/os_js.odin

@@ -250,11 +250,26 @@ current_thread_id :: proc "contextless" () -> int {
 	return 0
 }
 
-lookup_env :: proc(key: string, allocator := context.allocator) -> (value: string, found: bool) {
+@(require_results)
+lookup_env_alloc :: proc(key: string, allocator := context.allocator) -> (value: string, found: bool) {
 	return "", false
 }
 
-get_env :: proc(key: string, allocator := context.allocator) -> string {
-	value, _ := lookup_env(key, allocator)
-	return value
-}
+@(require_results)
+lookup_env_buffer :: proc(buf: []u8, key: string) -> (value: string, err: Error) {
+	return "", .Env_Var_Not_Found
+}
+lookup_env :: proc{lookup_env_alloc, lookup_env_buffer}
+
+@(require_results)
+get_env_alloc :: proc(key: string, allocator := context.allocator) -> (value: string) {
+	value, _ = lookup_env(key, allocator)
+	return
+}
+
+@(require_results)
+get_env_buf :: proc(buf: []u8, key: string) -> (value: string) {
+	value, _ = lookup_env(buf, key)
+	return
+}
+get_env :: proc{get_env_alloc, get_env_buf}