Add Input/InputBind/InputButton JS bindings, move default binds to JS

Adds a new script module exposing Input.bind/value/lastValue/isDown/
wasDown/pressed/released, InputBind.* (the inputbind_t action enum),
and InputButton.* (every named button in the compiled platform's
INPUT_BUTTON_DATA table, e.g. InputButton.SELECT on PSP).

Renames the PLATFORM global to Platform.current for consistency with
Time/Console/Input.

Uppercases every INPUT_BUTTON_DATA .name across inputlinux.c/
inputpsp.c/inputdolphin.c to match the InputButton.* JS constants
(inputButtonGetByName was already case-insensitive, so this doesn't
change matching behavior).

Moves the default key/button-to-action bindings out of
src/duskrpg/input/inputbindmap.h (deleted) and into
assets/scripts/input.js, branching on Platform.current instead of the
old per-platform #ifdefs. Wired in via require('./input.js') from
init.js.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

types/platform/platform.d.ts

@@ -0,0 +1,12 @@
+// Copyright (c) 2026 Dominic Masters
+//
+// This software is released under the MIT License.
+// https://opensource.org/licenses/MIT
+
+/**
+ * The engine's compile-time target system (see DUSK_TARGET_SYSTEM).
+ */
+declare const Platform: {
+  /** e.g. "linux", "psp", "wii", "gamecube", "knulli". */
+  readonly current: string;
+};