Adds range() function for lists

[matthew-dean]

While attempting to upgrade a Bootstrap Less port to use each() (seanCodes/bootstrap-less-port#12), I saw that there were still some gaps for porting Sass to Less code. Specifically, when looping X number of times based on a variable value. (This has been discussed in numerous issues, including as early as #249 and #2270.)

This PR fills a major gap by adding a very modest list-generating range() function modelled after Lodash / PHP / others. Coupled with each() it provides a native replacement for a Sass @for loop (along with letting you generate range lists for any other general purpose). This solves for loops without the syntactic burden of adding a new paradigm. It simply creates and populates a list in a declarative way.

Like Lodash, the function signature is:

range([start=1], end, [step=1])

Start and step are optional (with step requiring a start). Also, this adopts Less behavior of unit casting. That is, the "end" value's unit is applied to the output units. (Also, consistent with Less, lists are 1-based, so unlike Lodash, the default start value is 1.)

Meaning:

@list: range(10px, 30px, 10); // equivalent to writing 10px 20px 30px

Coupled with each(), this constructs for-style incremental loops.

@columns: range(4);  // 1 2 3 4

each(@columns, .(@val) {
  .col-@{val} {
    width: (100% / length(@columns));
  }
});

Outputs:

.col-1 {
  width: 25%;
}
.col-2 {
  width: 25%;
}
.col-3 {
  width: 25%;
}
.col-4 {
  width: 25%;
}

[seven-phases-max]

I can't review/test the code. But my +10 for the method itself (clear non-intrusive function w/o any new burdening syntax facilities).

[calvinjuarez]

Straight forward and simple. I like it a lot!

[calvinjuarez]

Failing test doesn't look to do with these changes at all.

[matthew-dean]

Thanks @seven-phases-max @calvinjuarez !