[Add]: Create new package stats/incr/nanwmean to handle NaN values

[ChaitanyaZunzurkar]

Resolves #{{issue number}}.

Description

This pull request introduces the @stdlib/stats/incr/nanwmean package, which computes the incremental weighted arithmetic mean while handling NaN values. Unlike @stdlib/stats/incr/wmean, this implementation specifically excludes NaN values in the mean calculation.

Key Changes:

• Adds a NaN-handling mechanism to the weighted arithmetic mean computation.
• Ensures that NaN values do not propagate through the mean calculation.
• Tests, documentation, and examples have been updated accordingly to reflect this behavior.

Related Issues

This pull request resolves #5743.

Tests and Benchmarks

All tests, examples, and benchmarks have been successfully run and validated:

• Unit tests for proper NaN handling.
• Examples demonstrating the behavior with NaN values.
• Benchmarks to assess performance.

Checklist

• Tests have been added or updated.
• Documentation and examples have been updated.
• Benchmarks have been validated.
• Pull request follows the contributing guidelines.
• Read, understood, and followed the contributing guidelines

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Coverage Report

Package	Statements	Branches	Functions	Lines
stats/incr/nanwmean	$\color{red}190/194$ $\color{green}+97.94\%$	$\color{red}12/15$ $\color{green}+80.00\%$	$\color{green}2/2$ $\color{green}+100.00\%$	$\color{red}190/194$ $\color{green}+97.94\%$

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

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