feat: add support for skew normal distribution PDF

[Prog-Jacob]

Resolves #492.

Description

What is the purpose of this pull request?

This pull request:

• Add support for skew normal distributions' PDF (probability density function).
• Add tests and benchmarks in JS and R for the newly added functions.
• Add documentations for all applied functions.

Related Issues

Does this pull request have any related issues?

This pull request:

• resolves #492

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.

No.

Checklist

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

• [x] Read, understood, and followed the contributing guidelines.

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@stdlib-js/reviewers

[kgryte]

/stdlib update-copyright-years

[kgryte]

@Planeshifter Looks like CI is failing due to a failure to install lintr.