refactor: strengthen kill input check

[antongolub]

continues #1339

• Tests pass
• Appropriate changes to README are included in PR

[Copilot]

Pull Request Overview

This PR strengthens input validation for the kill function to prevent objects with toString methods from being passed as process IDs, addressing a potential security/reliability issue.

• Enhanced the kill function's input validation to explicitly check for number and string types
• Added comprehensive test coverage for the new validation behavior
• Updated version numbers across the codebase

Reviewed Changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
src/core.ts	Enhanced kill function input validation to check for number/string types before regex validation
test/core.test.js	Added test case for object with toString method to verify rejection
package.json	Version bump to 8.8.3
src/versions.ts	Updated zx version to 8.8.3
build/index.cjs	Updated compiled version number
build/core.cjs	Updated compiled kill function with new validation logic
.size-limit.json	Adjusted size limits for bundle changes
.github/workflows/publish.yml	Added release tag validation workflow step

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