fix: preserve required properties when using onlyDefinedProperties

[mcollina]

Summary

Fixes #82

When using deepmerge({ onlyDefinedProperties: true }), the runtime correctly ignores undefined values from the source and preserves target values. However, the TypeScript types didn't reflect this behavior - merging Partial<T> into T would produce a type with optional properties even though at runtime those undefined values are skipped.

Before this fix:

interface Options { apiUrl: string; timeout: number }
type PartialOptions = Partial<Options>

const full: Options = { apiUrl: 'url', timeout: 100 }
const partial: PartialOptions = { timeout: 200 }

const result = deepmerge({ onlyDefinedProperties: true })(full, partial)
// result.apiUrl was typed as `string | undefined` ❌

After this fix:

const result = deepmerge({ onlyDefinedProperties: true })(full, partial)
// result.apiUrl is typed as `string` ✅
// Result is assignable to Options ✅

Changes

• Added new type definitions:

  • DeepMergeDefinedFn and DeepMergeAllDefinedFn function types
  • DeepMergeDefined<T, U> type that preserves required properties from target
  • IsMergeableObject<T> helper that works with Partial types (the original { [key: string]: unknown } check fails for optional properties)
  • Function overloads that return the new types when onlyDefinedProperties: true

• Added comprehensive type tests for:

  • Basic partial merging
  • Deep partial merging
  • all mode with onlyDefinedProperties

Test plan

• All 147 unit tests pass
• All type tests pass (npm run test:typescript)
• New type tests verify the fix works for the reported use case

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