[wasm] WasmApp.targets: Separate `obj`, and `bin` parts of the build process (#47253)

[wasm] WasmApp.targets: Separate `obj`, and `bin` parts of the build process (#47253)

We want to use a separate directory for intermediate build outputs, that aren't needed
in the app bundle, and reduce unclear internal dependencies during a build.

# TL;DR

## Changes:
1. `$(WasmBuildDir)` removed
2. Reasonable defaults set for most properties
3. To generate a wasm app for a project, the minimum you need to set:
    a. `$(WasmMainJS)`,
    b. and `@(WasmAssembliesToBundle)`

# Details:

Though it is a bit tricky, because the current targets assume:

- that they are being run after `Publish`
- that the publish directory has:
  - some required files copied over from the runtime pack (eg. `libmono*`),
  - and includes the assemblies
- that the targets emit all the intermediate output files like `driver.o`, or the bitcode files, into the same
   directory

- And there are assumptions about where to pick which files from (eg. whether to take `dotnet.wasm`
  from the runtime pack, or from the publish dir), based on unclear rules.

## What does this PR change?

- All the assets meant to be from the runtime pack (like `libmono*`, `icudt.dat`) are always taken
  only from the runtime pack
  - and this logic is moved out of the tasks, to the targets
- `$(WasmBuildDir)` is removed completely. Instead, we use an intermediate path based on `$(IntermediateOutputPath)`
  - and emit all the intermediate bits there, like the bitcode files
- Add reasonable defaults for various properties, like `$(WasmAppDir)`

Effectively:

1. To generate a wasm app for a project, the minimum you need to set:
    a. `$(WasmMainJS)`,
    b. and `@(WasmAssembliesToBundle)`

2. The targets don't depend on publish dir at all
    (in a future PR, we could remove triggering based on `Publish` target also)
* [wasm] WasmAppBuilder - move the list of native assets, and logic out

.. to the targets.

- New property: `NativeAssets`, populated by `@(WasmNativeAsset)`
- Remove property `MicrosoftNetCoreAppRuntimePackRidDir`
- Also, add the `icudt.dat` file from the targets

* [wasm] Simplify handling of dotnet.{js,wasm}

WasmAppBuilder has (non-obvious) logic to:

1. if AOT'ing, then use the *generated* dotnet.{js,wasm};
2. else use the one from the runtime pack

This depends on Publish having copied those files from the runtime pack
to the publish directory, and then comparing paths in the builder to
decide which one to use.

Instead, make this the intention obvious, and clear.

----
Commits:
* [wasm] Always get the native libs from the runtime pack (eg.libmono*)

We were getting these from the publish directory, instead we can get
them directly from the runtime pack.

This includes icudt.dt, and dotnet.timezones.blat .

* [wasm] MonoAOTCompiler: add `OutputDir` property

.. where we can emit the generated native files. Since these files are
meant only for generating the final `dotnet.wasm`, we don't want them to
put them in the bin directory.

* [wasm] Use existing list of assemblies - @(_WasmAssemblies)

.. instead of trying to find them in the build dir. This build directory
will become a directory for intermediate build output in upcoming
commits.

* [wasm] Replace $(WasmMainAssemblyPath) with $(WasmMainAssemblyFileName)

- Instead of having a special $(WasmMainAssemblyPath), and then adding
  it to the wasm assemblies ourselves
  - let the user project add all the relevant assemblies to
    `@(WasmAssembliesToBundle)`, which is usually as simple as
    `$(OutDir)\*.dll`.

- This helps to simplify lot of things.
- And we just need the main assembly filename for generating the
  run-v8.sh script.

* [wasm] Rename WasmBuildDir -> _WasmIntermediateOutputPath

Based on the changes in previous commits, we can now remove
`$(WasmBuildDir)`, and replace that with an internal
`$(_WasmIntermediateOutputPath)`. This path will have all the build
artifacts generated that aren't required in the app bundle.

Earlier, we were using the publish directory for that, which resulted in
it being littered with unncessary files, and files getting copied to the
app bundle from unclear sources, and for non-obvious reasons.

* [wasm] add default value for $(WasmAppDir)

* [wasm] WasmApp.targets - misc cleanup

* [wasm] WasmAppBuilder: validate Assemblies property

* [wasm] WasmTestRunner - rename TestAssembly->TestAssemblyFileName, to correctly reflect the value

* [wasm] Fix make clean, for samples

* [wasm] WasmApp.targets: Add new $(MonoAotCrossCompilerPath)

* [wasm] update comments/docs

* Address review feedback from @mdh1418