feat: allow calling RunBazelisk with custom stderr

[sitaktif]

This allows library users to control what happens with stderr (e.g. write it to a file or buffer instead of the process' stderr).

[Copilot]

Pull request overview

This PR adds support for custom stderr handling in Bazelisk's Go library API, allowing library consumers to redirect stderr output to custom writers instead of always using the process's stderr. This enhancement improves the flexibility of Bazelisk when used as a library.

Changes:

• Added new public API function RunBazeliskWithArgsFuncAndConfigAndOutAndErr that accepts both stdout and stderr writers
• Updated internal functions makeBazelCmd and runBazel to accept and handle custom stderr writers
• Maintained backward compatibility by having existing functions delegate to the new function with nil for stderr

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[Copilot]

The new stderr parameter functionality in RunBazeliskWithArgsFuncAndConfigAndOutAndErr lacks test coverage. Consider adding a test that verifies stderr can be redirected to a custom writer, similar to how stdout redirection should work. This would help ensure the feature works as intended and prevents regressions.

[Copilot]

The documentation incorrectly states that both stdout and stderr are written to "the passed writer" (singular), but the function signature shows two separate writers for stdout and stderr respectively. The documentation should clarify that stdout is written to the first writer and stderr is written to the second writer, or say "the passed writers" (plural).

Suggested change

	// repositories and config, writing its stdout and stderr to the passed writer.
	// repositories and config, writing its stdout to the first passed writer and stderr to the second.