feat: add number/float16/base/signbit

[Neerajpathak07]

Resolves none.

Description

What is the purpose of this pull request?

This pull request:

• Adds number/float16/base/signbit

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• none

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{{TODO: add disclosure if applicable}}

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@stdlib-js/reviewers

[Neerajpathak07]

/stdlib update-copyright-years

[stdlib-bot]

Coverage Report

Package	Statements	Branches	Functions	Lines
number/float16/base/signbit	$color{green}178/178$ $color{green}+100.00%$	$color{green}7/7$ $color{green}+100.00%$	$color{green}2/2$ $color{green}+100.00%$	$color{green}178/178$ $color{green}+100.00%$

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

[Neerajpathak07]

Given that Repl error is not caused to know changes related to this PR hence I'll go ahead and open this for Review!

[Neerajpathak07]

testing against float16/smallest-subnormal val to test underflow and bit-level correctness.
As it is the smallest non-zero magnitude number that still carries a sign bit.

[Neerajpathak07]

Since toWord converts to an unsigned 16-bit integer returning the values after shift off all bits is why I went with using 15. Also to make sure the number bits do not overflow.