feat: add ng add schematic support

[valeriocomo]

Closes #3

What

Adds an ng-add schematic so cashew can be installed and configured with a single command:

ng add @openng/cashew

The CLI installs the package (saved to dependencies) and the schematic automatically registers provideHttpCache() and the cache interceptor in the application config — no manual wiring needed.

How it works

The schematic covers the following scenarios:

• Fresh app (no provideHttpClient) — adds provideHttpClient(withInterceptors([withHttpCacheInterceptor()])) and provideHttpCache() via addRootProvider from @schematics/angular/utility, which transparently supports both standalone (app.config.ts) and NgModule-based apps.
• Existing provideHttpClient(...) — merges via TypeScript AST: appends a withInterceptors([withHttpCacheInterceptor()]) feature argument, or appends withHttpCacheInterceptor() to an already existing withInterceptors([...]) array, merging import statements.
• Legacy HttpClientModule / unparseable code — never makes an ambiguous edit: still adds provideHttpCache() and logs a warning with the manual setup snippet.
• Idempotent — running it twice produces no duplicates.

Only public, stable APIs are used (@angular-devkit/schematics, addRootProvider/readWorkspace, typescript), available since @schematics/angular 16.1 — safe across the whole supported peer range (@angular/core >=17). No private deep imports, following the Angular Material approach.

Build & CI changes

• schematics/ folder compiled with its own tsconfig.json (CommonJS) into dist/openng/cashew/schematics via the new build:schematics script, hooked into postbuild:lib; collection.json/schema.json shipped through ng-packagr assets.
• Library package.json now declares "schematics" and "ng-add": { "save": "dependencies" }.
• New test:schematics script (Jest + SchematicTestRunner, Node environment, 9 tests) added as a CI step.
• README now documents ng add @openng/cashew as the recommended installation path.

Testing

• ✅ 9/9 schematic tests, 46/46 existing library tests.
• ✅ End-to-end: packed the built library (npm pack), generated a fresh app with Angular CLI 20, ran ng add from the tarball — verified both a clean app and an app with a pre-existing provideHttpClient(withFetch(), withInterceptors([authInterceptor])); the resulting apps build successfully.

[valeriocomo]

hi @geromegrignon. I've already had experience in implementing this feature in the past.

I've written some tests to verify the behaviour of this schematic.

I'd be interested to know if there are other effective ways to test this feature.

[geromegrignon]

Thanks for your contribution!