feat: add C implementation for math/base/special/truncn

[gunjjoshi]

Description

What is the purpose of this pull request?

This pull request:

• adds C implementation for math/base/special/truncn.

double stdlib_base_truncn( const double x, const int32_t n );

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves a part of [RFC]: Add C implementations to base special math functions (tracking issue) #649.

Questions

Any questions for reviewers of this pull request?

No.

Other

Any other information relevant to this pull request? This may include screenshots, references, and/or implementation notes.

Similar to what we did in roundb, I have remove some test cases here too, which were not applicable.

Checklist

Please ensure the following tasks are completed before submitting this pull request.

• Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[Planeshifter]

One small request, otherwise this looks great!