feat: support build without config file

[fi3ework]

Summary

support run command without config file.

when no config is present, Rslib will use a fallback lib object accept options from CLI.

Related Links

Checklist

• Tests updated (or not required).
• Documentation updated (or not required).

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[Copilot]

Pull Request Overview

This PR enables Rslib to build projects without requiring a configuration file. When no rslib.config.* file is present, the CLI falls back to a default configuration with a single library entry and accepts all build options from command-line arguments.

Key Changes:

• Modified config resolution to return undefined instead of throwing an error when no config file is found
• Added fallback logic to create a default lib configuration when no config file exists
• Updated tests to verify building without a config file works correctly

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
packages/core/src/config.ts	Changed resolveConfigPath to return undefined instead of throwing when config is missing; loadConfig now returns empty lib array when no config found
packages/core/src/cli/initConfig.ts	Added logic to initialize default lib config when configFilePath is undefined; filters out undefined values from watchFiles
tests/integration/cli/build/build.test.ts	Updated test from expecting an error to verifying successful build output without config file
tests/integration/cli/build/no-config/src/index.ts	Changed test data from string to numeric constant
website/docs/en/guide/basic/cli.mdx	Added documentation note explaining fallback behavior when config file is absent
website/docs/zh/guide/basic/cli.mdx	Added Chinese translation of fallback behavior documentation

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