refactor: simplify ProcessPromise._snapshot inners

[antongolub]

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

[Copilot]

Pull Request Overview

This PR refactors the ProcessPromise internal structure by consolidating context management into a unified _snapshot object approach. It simplifies the class by removing multiple individual private properties and centralizing configuration state.

• Replaces individual properties (_cmd, _from, _stdio, etc.) with a consolidated _snapshot object approach
• Introduces a Snapshot type and getSnapshot helper function for better type safety
• Removes redundant property accessors and simplifies the promise resolution logic

Reviewed Changes

Copilot reviewed 8 out of 11 changed files in this pull request and generated 3 comments.

Show a summary per file

File	Description
src/core.ts	Major refactoring of ProcessPromise class to use consolidated snapshot approach, introduces Snapshot type and getSnapshot helper
test/core.test.js	Updates test setup to use new snapshot structure and adds test for kill() with empty pid
src/cli.ts	Minor code reorganization - moves export statement and simplifies conditional logic
package.json	Version bump to 8.7.2 and esbuild dependency update
build/ files	Generated build artifacts reflecting the core changes
.size-limit.json	Updated size limits reflecting bundle size changes

[Copilot]

The change from p == undefined to p == null alters the behavior. The original check with == undefined would catch both undefined and null values, while == null only catches null and undefined. Consider using strict equality === null || === undefined or reverting to the original logic if the intention was to catch both falsy values.

Suggested change

	if (pieces.some((p) => p == null))
	if (pieces.some((p) => p === null || p === undefined))