This PR modernizes several parts of the codebase by:

1. Replacing arguments with rest parameters (...objs) in the merge function.
2. Replacing indexOf(...) !== -1 with .includes(...) for better readability.
3. Replacing indexOf(...) === 0 with .startsWith(...) in the XHR adapter.

These changes align with modern JavaScript standards already present in the Axios 1.x codebase.

Summary by cubic

Modernizes utils and the XHR adapter to ES6 (rest params, .includes, .startsWith) for clarity and consistency with v1.x. Synced with the latest v1.x; no behavior or API changes.

Description

• Switched merge to rest params (keeps caseless/skipUndefined).
• Replaced indexOf with .includes(...) across utils/XHR, including the restricted props check in freezeMethods.
• Used .startsWith('file:') for file: URL detection; protocol allowlist now uses .includes(...).
• Docs: no user-facing changes; consider updating /docs/ coding standards to prefer ES6 patterns.
• Semantic version impact: patch (refactor only).

Testing

• No tests changed; existing tests for XHR protocol handling and utils.merge are sufficient.

^{Written for commit 02dd117. Summary will update on new commits.}