refactor: replace @epic-web/invariant with simple code

[hyperz111]

What:

Replace @epic-web/invariant with simple code. (This is like #266)

Why:

We only use one function (invariant) from @epic-web/invariant which is can be reached with this simple code:

if (!condition) {
    throw new Error(/* ... */)
}

By doing that, cross-env now only use 1 dependency (cross-spawn).

How:

Replace @epic-web/invariant with simple code like on top.

Checklist:

• Documentation
• Tests
• Ready to be merged
  

[coderabbitai]

Walkthrough

The changes remove the @epic-web/invariant dependency from package.json and replace its single usage in src/index.ts with an inline error throw statement for command validation.

Changes

Cohort / File(s)	Change Summary
Dependency Removal package.json	Removed @epic-web/invariant from the dependencies section
Runtime Validation Refactor src/index.ts	Replaced invariant() utility call with inline if (!parsedCommand) throw new Error('Command is required') statement

Estimated code review effort

🎯 1 (Trivial) | ⏱️ ~5 minutes

• Straightforward dependency removal with corresponding inline replacement
• Single validation point changed with identical behavior
• No alterations to exported APIs or function signatures

Poem

🐰 A dependency falls away,
Invariant thought now simplified,
The throw speaks clear as day—
Direct and verified.
Less magic, more true. ✨

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ Passed checks (2 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'refactor: replace @epic-web/invariant with simple code' accurately and specifically describes the main change: replacing a dependency with inlined code.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[hyperz111]

@kentcdodds, what about this?

[codecov]

Codecov Report

❌ Patch coverage is 33.33333% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 98.71%. Comparing base (2ff1d88) to head (ec26983).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/index.ts	33.33%	2 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main     #267      +/-   ##
===========================================
- Coverage   100.00%   98.71%   -1.29%     
===========================================
  Files            4        4              
  Lines          155      156       +1     
  Branches        52       53       +1     
===========================================
- Hits           155      154       -1     
- Misses           0        2       +2     

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