chore(deps): replace lodash with native JS equivalents

[roli-lpci]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

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

What is the current behavior?

The project depends on lodash (and @types/lodash) across three packages (jest, custom-webpack, root) for a handful of utility functions: mergeWith, merge, get, pick, isArray, differenceWith, keyBy, and remove.

Lodash is a CJS-only dependency that hasn't been updated in four years, increases bundle size due to lack of tree-shaking support, and can cause optimization bailouts.

Issue Number: #1904

What is the new behavior?

All lodash functions replaced with native JS equivalents:

lodash function	Native replacement	Location
mergeWith	Custom deepMergeWith with customizer callback	merge-schemes.ts, jest-configuration-builder.ts
merge	Custom deepMerge (recursive, plain-object aware)	webpack-config-merger.ts
get	Custom getByPath with dot/bracket path support	custom-webpack-builder.ts
pick	Object.fromEntries + filter	default-config.resolver.ts
isArray	Array.isArray	jest-configuration-builder.ts
differenceWith	Array.filter + some	webpack-config-merger.ts
keyBy	for...of loop building a record	webpack-config-merger.ts
remove	Array.filter (reassignment)	custom-esbuild-schema.spec.ts

lodash and @types/lodash removed from root package.json, packages/jest/package.json, and packages/custom-webpack/package.json.

All existing unit tests pass unchanged (33 tests across jest + custom-webpack packages).

Does this PR introduce a breaking change?

[ ] Yes
[x] No

Other information

The deep merge helpers (deepMergeWith in merge-schemes.ts and jest-configuration-builder.ts, deepMerge in webpack-config-merger.ts) are intentionally inlined in each file rather than shared, since they have slightly different signatures and the merge-schemes.ts build script runs outside the package dependency graph.

The schema merging __REPLACE__ / __DELETE__ marker behavior is preserved exactly as documented in AGENTS.md.