feat: add Wald distribution mean package

[manit2004]

Resolves #209

Description

This is a WIP pull request. Currently the core functionalities of Wald Distribution mean package has been implemented. Feedback needed on this implementation.

Tests, docs, examples and benchmark is yet to added, pending approval of the core implementation.

Related Issues

Does this pull request have any related issues?

This pull request has the following related issues:

• RFC: add the Wald distribution #209

Questions

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No.

Other

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I have used this Wiki page as a reference.

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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]

First examples should always showcase valid inputs (this is something that should be addressed in various files).

[Planeshifter]

Suggested change

	* Returns the expected value for a wald distribution with mean `mu` and standard deviation `sigma`.
	* Returns the expected value for a Wald distribution with mean `mu` and standard deviation `sigma`.

[Planeshifter]

Suggested change

	* Evaluates the expected value for a Wald distribution with mean `mu` and standard deviation `sigma`.
	* Returns the expected value for a Wald distribution with mean `mu` and standard deviation `sigma`.

[Planeshifter]

As in #9324, the Wald distribution has a second shape parameter lambda and not standard deviation sigma, so this will have to be updated across this PR.

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
stats/base/dists/normal/mean	$\color{green}175/175$ $\color{green}+0.00%$	$\color{green}9/9$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}175/175$ $\color{green}+0.00%$
stats/base/dists/wald/mean	$\color{green}184/184$ $\color{green}+0.00%$	$\color{green}10/10$ $\color{green}+0.00%$	$\color{green}2/2$ $\color{green}+0.00%$	$\color{green}184/184$ $\color{green}+0.00%$

The above coverage report was generated for the changes in this PR.

[Planeshifter]

Thanks for the edits!

The PR is currently missing README.md, benchmarks, and also contains extraneous changes to normal/mean that should be reverted.

[Planeshifter]

Suggested change

	If provided `σ <= 0` or `λ <= 0` the function returns `NaN`.
	If provided `μ <= 0` or `λ <= 0` the function returns `NaN`.

[Planeshifter]

Descriptions need to be aligned here:

Suggested change

	* @param mu mean
	* @param lambda shape parameter
	* @return expected value
	* @param mu mean
	* @param lambda shape parameter
	* @return expected value