Extend length method to work with strings

[iaman]

Extends the length method to allow it to check string length as well; perhaps not terribly useful, but it seems odd to have a length method that doesn't work with a basic type that has a length property.

[kizu]

Thanks for the PR! While this can be a useful feature, it can break someone's code due to how lists work in Stylus: someone could use the current behaviour of length() to check if the input is a list, expecting a single string to be 1, for example.

While we don't have proper arrays, I'd propose to add a new bif for this, named strlength for example, so there won't be conflicts with the existing code.

Also, it would be nice if this bif did type coersion, so, for example, it could also accept indents and function names as input and return their string length.

[iaman]

@kizu Sounds good, I'll get hacking on it later today :)

[TerraTech]

For those that TLDR; length() was extended to support string type in release: v0.54.6
https://github.com/stylus/stylus/releases/tag/0.54.6

[As an aside, it hasn't been added to the Stylus documentation as of yet.]