Fix MergeDeep to remove unintentional undefined from optional properties (#1314)

[taiyakihitotsu]

Close #1314

(Updated the entire comment.)

This PR is a small change, but it consists of two parts.

Part 1

import type {MergeDeep} from 'type-fest';

interface Pokemon {
      name: string;
      type: string;
    }

MergeDeep<Pokemon, Partial<Pokemon>> should be equivalent to Partial<Pokemon> according to this comment in merge-deep.d.ts:

type-fest/source/merge-deep.d.ts

Line 386 in c671aa2

- Properties that exist in both objects are merged if possible or replaced by the one of the source if not.

However, before this PR, the result type was:

{
  name?: string | undefined;
  type?: string | undefined;
}

which causes the following error (in the issue):

error TS2322: Type '{ name?: string | undefined; type?: string | undefined; }' is not assignable to type 'Pokemon'.

(Note: The above is a direct quote from an issue comment. In this case, it seems appropriate to type it as Partial<Pokemon> instead of Pokemon.)

This extra | undefined is unnecessary here.
This PR removes that redundancy with the following change:
https://github.com/sindresorhus/type-fest/pull/1319/files#diff-5ce63d84bd0497ef6dd72ef89147e7bf80f464a1185b1a34139c6c1cb6c37ee5R36

Part 2

After the change in Part 1, this test doesn't pass:

type-fest/test-d/merge-deep.ts

Line 325 in c671aa2

expectType<true>({} as IsEqual<RecordPartial, testRecordReplacedByOptionalRecord>);

Required is added to fix this:
https://github.com/sindresorhus/type-fest/pull/1319/files#diff-5ce63d84bd0497ef6dd72ef89147e7bf80f464a1185b1a34139c6c1cb6c37ee5R61

Note

The issue described above may be related to PR #1209, which has been suspended since August 31.
I intentionally did not include any changes related to that PR in order to keep this change minimal.

If these changes are considered inappropriate due to the context of PR #1209, I’m fine with closing this PR.