Add space before units in toString() metod

[ItamarKieslerFabric]

Hi, thank you for maintaining this awesome package.

The official recommendation is to include a space between the quantity and the unit for all units except superscript units such as degrees. Can it please be added to toString() behaviour?

// Current version:
convert(5000, 'feet').to('best').toString(); // '1.524km'
// Desired behavior:
convert(5000, 'feet').to('best').toString(); // '1.524 km'
//                                        space here: ^

Thanks.

Sources:
https://www.nist.gov/pml/special-publication-811/nist-guide-si-chapter-7-rules-and-style-conventions-expressing-values#7.2
https://pml.nist.gov/cuu/Units/checklist.html (paragraph 15)
https://www.writing-skills.com/spaces-units-60-second-fix

[jonahsnider]

Thank you for taking the time to link some resources about this! I'll take a look at those tomorrow and should be able to resolve this then as well.