[browser] Add application environment to boot config

[maraf]

• Add MSBuild property to define application environment.
• Prefer value defined by JavaScript API over the value in boot config.
• Unrelated: Use unicode character in Wasm.Build.Tests path only on CI by default, because it complicates copy-pasting paths from terminal locally

Contributes to dotnet/aspnetcore#59456

PR Overview

This PR introduces functionality for defining and overriding the application environment across boot configuration, MSBuild, and JavaScript API. Key changes include adding a new MSBuild property and associated task changes, updating the JavaScript boot configuration loader to prefer the user-defined environment, and extending tests to cover the new behavior.

Reviewed Changes

File	Description
src/mono/wasm/Wasm.Build.Tests/AppSettingsTests.cs	Updates tests to validate environment selection based on publish/build and overrides from MSBuild/JavaScript.
src/mono/browser/runtime/loader/config.ts	Updates boot config loading logic to prefer a user-defined application environment from the JavaScript API.
src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/BootJsonData.cs	Adds the applicationEnvironment property to the boot config data.
src/mono/wasm/testassets/WasmBasicTestApp/App/wwwroot/main.js	Adjusts the call to set the application environment based on query parameters.
src/tasks/Microsoft.NET.Sdk.WebAssembly.Pack.Tasks/GenerateWasmBootJson.cs	Integrates the new MSBuild ApplicationEnvironment property into the boot JSON generation.
src/mono/wasm/Wasm.Build.Tests/Templates/WasmTemplateTestsBase.cs	Refactors the test asset copying method to better handle unicode path handling on local systems.
src/mono/wasm/testassets/WasmBasicTestApp/README.md	Updates the referenced test assets path in the documentation.

Copilot reviewed 8 out of 8 changed files in this pull request and generated no comments.

[maraf]

/ba-g OutOfMemoryException is unrelated