Add basic thread pool example

[TosterHolokoster]

For the issue #103. This example shows basic usage of the thread pool.

I aimed to make the example as beginner-friendly as possible. However, it means that the example doesn't demonstrate the full power of the thread poll. Therefore, it is a basic example. I hope at some point someone (maybe me) will add an advanced example. Something like a loop of waiting, popping complete tasks, and adding new ones in their place.

Checklist before submitting:

• This example has been added to .github/workflows/check.yml (for automatic testing)
• This example compiles cleanly with flags -vet -strict-style -vet-tabs -disallow-do -warnings-as-errors
• This example follows the core naming convention: https://github.com/odin-lang/Odin/wiki/Naming-Convention (exception can be made for ports of examples that need to match 1:1 to the original source).
• By submitting, I understand that this example is made available under these licenses: Public Domain and Odin's BSD-3 license. Only for third-party dependencies are other licenses allowed.

[karl-zylinski]

Thanks for the PR. I've added a new style convention document, please read it and correct the style in this example: https://github.com/odin-lang/examples/wiki/Naming-and-style-convention

[TosterHolokoster]

I updated the code to comply with the new style convention. However, I left a separate declaration/definition for some variables. It helps explain things and splits the code into bit-sized pieces. Hope you're fine with it.

[karl-zylinski]

Sorry for the delay. I'm looking for someone to review this, since it is not my area of expertise.

[karl-zylinski]

Thank you for the PR!