feat: improve static.serveIndex defaults

[snitin315]

• This is a bugfix
• This is a feature
• This is a code refactor
• This is a test update
• This is a docs update
• This is a metadata update

For Bugs and Features; did you add new tests?

Yes.

Motivation / Use-Case

Resolve TODO.

• Check if 'serveIndex' property is defined in 'optionsForStatic'
• If 'serveIndex' is a boolean and true, use default 'serveIndex'
• If 'serveIndex' is an object, merge its properties with default 'serveIndex'
• If 'serveIndex' is neither a boolean true nor an object, use it as-is

Breaking Changes

Yes, updated defaults.

Additional Info

No.

[codecov]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.01% 🎉

Comparison is base (a87dc53) 92.04% compared to head (551e612) 92.05%.

Additional details and impacted files

@@ Coverage Diff @@ ## next #4968 +/- ## ========================================== + Coverage 92.04% 92.05% +0.01%  ========================================== Files 15 15 Lines 1571 1573 +2 Branches 595 597 +2 ========================================== + Hits 1446 1448 +2  Misses 116 116 Partials 9 9 

Files Changed	Coverage Δ
lib/Server.js	94.26% <100.00%> (+0.01%)	⬆️

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