feat(@angular/build): add experimental vitest unit-testing support

[clydin]

When using the application build system via the @angular/build package (default for new projects starting in v20), a new experimental unit-test builder is available that initially uses vitest. This experimental system is intended to provide support for investigation of future unit testing efforts within the Angular CLI. As this is experimental, no SemVer guarantees are provided, the API and behavior may change, and there may be unexpected behavior. Available test runners may be added or removed as well.

The setup is somewhat different than the previous unit-testing builders. It uses a similar mechanism to that of the dev-server and requires a buildTarget option. This allows the code building aspects of the unit-testing process to leverage pre-existing option values that are already defined for development. If differing option values are required for testing, an additional build target configuration specifically for testing can be used.

The current vitest support has multiple caveats including but not limited to:

• No watch support
• jsdom based testing only (jsdom must be installed in the project)
• Custom vitest configuration is not supported
• Code coverage is not yet available
• Reporter customization is not yet available

An example configuration that would replace the test target for a project is as follows:

"test": {
    "builder": "@angular/build:unit-test",
    "options": {
        "tsConfig": "tsconfig.spec.json",
        "buildTarget": "::development",
        "runner": "vitest"
    }
}