Add usage example for JsPromise wrapper

[lvllvl]

This Pull Request fixes/closes #2761 .

It adds the following:

• a new file --> boa_examples/src/bin/jspromise.rs
• the code example is taken from boa_engine/src/object/builtins/jspromise.rs (line 115-155)

[Razican]

The example looks good, but looking at other examples, we should be adding more comments through the code, explaining what each step does.

Also, is there any other API usage we can show?

[lvllvl]

Added comments to the code.

Do you mean that this example should use more of the api or that there should be another jspromise example in addition to this one?

[jedel1043]

Probably that the example should use more methods of JsPromise.

[codecov]

Codecov Report

Merging #2804 (1dc7414) into main (24a4770) will decrease coverage by 0.02%.
The diff coverage is 0.00%.

❗ Current head 1dc7414 differs from pull request most recent head 4153843. Consider uploading reports for the commit 4153843 to get more accurate results

@@            Coverage Diff             @@
##             main    #2804      +/-   ##
==========================================
- Coverage   51.30%   51.28%   -0.02%     
==========================================
  Files         416      417       +1     
  Lines       41193    41204      +11     
==========================================
  Hits        21133    21133              
- Misses      20060    20071      +11     

Impacted Files	Coverage Δ
boa_examples/src/bin/jspromise.rs	0.00% <0.00%> (ø)

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[jedel1043]

@lvllvl Do you have any progress for this? If you don't have the time to work on this, that's also okay :)