fix(format): preserve built-in fields when partially overriding a formatter

[zerone0x]

Issue for this PR

Closes #14857

Type of change

• Bug fix

What does this PR do?

When a user overrides a built-in formatter's extensions without also specifying command, the entire override was silently discarded and the original built-in (including all default extensions) remained unchanged.

Root cause: The merge order was wrong. The code spread command: [] as an empty stub before ...item, then passed the combined object as the override to mergeDeep. Since the user didn't provide command, the stub [] was used — which then replaced the built-in command array. The command.length === 0 guard fired, skipping the formatter entirely.

Fix: Change the merge order to: defaults < built-in < user config

// Before (broken): stub defaults override built-in fields the user didn't specify
mergeDeep(formatters[name] ?? {}, { command: [], extensions: [], ...item })

// After (fixed): built-in values fill in what the user didn't override
{
  command: [],
  extensions: [],
  enabled: async () => true,
  ...(formatters[name] ?? {}),  // built-in overrides defaults
  ...userOverride,               // user config overrides built-in (only provided fields)
  name,
}

This means a user can now write:

"formatter": {
  "prettier": {
    "extensions": [".js", ".ts"]
  }
}

…and prettier's built-in command is preserved while only the extensions are narrowed.

How did you verify your code works?

• Traced the logic manually for: (a) built-in override with only extensions, (b) override with only command, (c) override with both, (d) new custom formatter, (e) disabled: true — all cases behave correctly.
• Removed the now-unused mergeDeep import from remeda.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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