feat(schematics): ng-add for custom-esbuild/webpack + jest migration schematics

[just-jeb]

PR Checklist

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

[ ] Bugfix
[x] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[ ] Build related changes
[ ] CI related changes
[ ] Documentation content changes
[ ] Other... Please describe:

What is the current behavior?

Users must manually update angular.json and package.json when adding @angular-builders/custom-esbuild or @angular-builders/custom-webpack to their project. Users upgrading @angular-builders/jest across major versions have no automated migration support.

Issue Number: N/A

What is the new behavior?

Three additive schematics features. Targeted at the next major aligned with Angular 22 (not yet released) — see "Versioning" below.

Commit 1: feat(custom-esbuild): add ng-add schematic

ng add @angular-builders/custom-esbuild now:

• Rewrites architect.build.builder in angular.json to @angular-builders/custom-esbuild:application
• Adds @angular-builders/custom-esbuild to devDependencies in package.json
• Schedules NodePackageInstallTask (skippable via --skipInstall)

New files: src/schematics/collection.json, src/schematics/ng-add/{index.ts,schema.json,schema.ts}, tsconfig.schematics.json. Mirrors the jest ng-add schematic from #2240.

Commit 2: feat(custom-webpack): add ng-add schematic

Same structure as commit 1, for @angular-builders/custom-webpack:

• Rewrites architect.build.builder to @angular-builders/custom-webpack:browser
• Adds @angular-builders/custom-webpack to devDependencies
• Schedules NodePackageInstallTask

Commit 3: feat(jest): add ng-update migration schematics v18-v21

ng update @angular-builders/jest now applies migrations across the v18→v21 chain:

Migration	Version jump	What it does
v18	v17→v18	Updates tsconfig.spec.json types from jasmine to jest; emits guidance on setup file rename
v19	v18→v19	Emits guidance on the zone.js setup change to explicit setupZoneTestEnv()
v20	v19→v20	No-op (no breaking changes; completes the migration chain)
v21	v20→v21	Sets zoneless: false in angular.json for projects using the jest builder (v21 defaults to zoneless: true); warns about removed global mocks

New files: src/schematics/migrations.json, src/schematics/migrations/{v18,v19,v20,v21}/index.ts, tsconfig.schematics.json.

Implementation notes

• All schematics compile with a separate tsconfig.schematics.json (CommonJS, Node moduleResolution) and are copied to dist/schematics/ as part of each package's build.
• The custom-esbuild/custom-webpack schema.ts files for schematics are hand-authored (quicktype only generates the builder option schemas, not schematics schemas) and kept in sync manually.
• tsconfig.schematics.json uses ignoreDeprecations: "6.0" to allow moduleResolution: Node, required by @angular-devkit/schematics; the repo root tsconfig uses Node16, which is incompatible with Angular's schematics runtime.
• custom-esbuild and custom-webpack .gitignore add a !src/schematics/**/schema.json exception (those packages ignore **/schema.json globally because builder schemas are generated at build time).

Versioning

The schematics are purely additive (no existing builder behaviour changes), but they are intended to land with the v22-aligned major rather than the current 21.x line, so the ng-add/ng-update surface ships together with the Angular 22 support. Should be merged after #2240 (shared packages/jest/package.json and jest tsconfig.schematics.json edits will need a rebase).

Does this PR introduce a breaking change?

[ ] Yes
[x] No

New schematics are purely additive. No existing builder behaviour is changed.

[just-jeb]

Superseded by #2267, which consolidates ng-add/ng-update schematics for all builders (jest, custom-esbuild, custom-webpack) in a single PR targeting release/v22.