test: add full coverage

[mateonunez]

Full Coverage

This PR aims to bring a full-covered code by tests.

To do this I've made some changes to make the code more testable like: error handling and edge cases.

This PR is labeled as:

Choose only one and remember to select the label even on the right.

• Feature
• Bug
• Other

---------------|---------|----------|---------|---------|-------------------
File           | % Stmts | % Branch | % Funcs | % Lines | Uncovered Line #s 
---------------|---------|----------|---------|---------|-------------------
All files      |     100 |      100 |     100 |     100 |                   
 constants.js  |     100 |      100 |     100 |     100 |                   
 dictionary.js |     100 |      100 |     100 |     100 |                   
 stdlib.js     |     100 |      100 |     100 |     100 |                   
 utils.js      |     100 |      100 |     100 |     100 |                   
---------------|---------|----------|---------|---------|-------------------

[airscripts]

Thank you so much for your contribute @mateonunez!
I've made some edits in order to not bother you with them.

I've removed one of the tests, since it exposes a flaw that I've seen inside stringify function. I'll try to fix it in another PR.
Aside of this, I've added one more test for the compile function and started testing utils.
For this last one, I think it's needed a spy function for the process.exit()'s one...and this was the reason on why I've avoided the try/catch system on the cli's root file.

Hope you had fun with this and hopefully even a smile for this project. Thank you kindly!