There were some implicit casts in the standard codec implementation that
didn't show up on Linux, but do on Windows.

* Allow specifying both Dart and non-Dart fixtures in engine unittests.

This fixes numerous issues in the way in which fixtures were managed
in the engine unit-tests.

* Instead of only being able to specify Dart fixtures, unit-tests may specify
  non-Dart fixtures as well. These are simply copied over to the fixtures
  directory known to the unit-test at runtime.
* An issue where numerous Dart files could be given to the kernel snapshotter
  has been addressed. It was anticipated that such a (legal) invocation to the
  kernel snapshotter would produce a snapshot with the contents of all the Dart
  files added to the root library. This is incorrect and the behavior in this
  case is undefined.
* Dart files referenced by the main Dart file are correctly tracked via a
  depfile.
* The snapshotter arguments have been cleaned up to get rid of unused
  arguments (`—strong`) and  the use of the VM product mode argument has been
  corrected to no longer depend on the Flutter product mode.

Corects a bnuch of typeos throughout teh engien codebsae. Also makes
a couple minor Commonwealth -> US spelling adjustments for consistency
with the rest of Flutter's codebase.

Made use of `misspell` tool:
https://github.com/client9/misspell

- Makes json_method_codec.cc compatible with the last stable RapidJSON release.
- Allows removing the GTK dependency with a compile flag.
- Fixes a missing break in a switch flagged by some toolchains.

Plugins may need to be able to access functions affecting the GLFW
window (e.g., a plugin to resize the window). This restructures the API
to create a distinction at both the C and C++ level between the window
controller, which provides access to high-level behaviors driving the
Flutter application, and the window, which provides access to functions
to affect the UI state of the window (i.e., wrapped GLFWwindow
functions).

Also provides a PluginRegistrar extension for plugins that need access
to GLFW-specific functionality.

Builds the unit test on all platforms, and adds them to the aggregate test script.

Adds StandardMethodCodec support to the C++ client wrapper. This makes it
substantially easier to add Windows and Linux support for existing plugins, as
StandardMethodCodec is the default plugin protocol.

Fixes flutter/flutter#30670

Does not include extensibility for the codec, which will be added later.

Adds a type that can hold any of the types corresponding to the Dart types
that are supported by the standard message channel codec. This provides
the foundation for adding standard message codec support for the C++
desktop shells (flutter/flutter#30670).

Enables the build of the Windows shell, based on the same GLFW code used
by the current Linux shell.

Changes include:
- File structure
- Header guards
- Include paths
- Namespaces
- Integration with the engine's GN build
- Conversion from jsoncpp to rapidjson
- Style and clang-format adjustment to match engine repository