Line size legend component

[rjimenezda]

Fix #612 Fix #615

Created the base <as-legend-line-size> component and added the following aliases:

• <as-legend-size-bins-line>
• <as-legend-size-continuous-line>

And now, <as-legend-size-bins> will render it if the type is 'line', and so will <as-legend-size-continuous>

~Unrelated tests are failing despite not having changed anything :thinking:~ A rerun fixed it

[rjimenezda]

I uploaded the continuous one.

This component is slightly different from the rest because it's done with SVG. so it's less flexible size-wise. You can specify a size property (300px by default), and even a factor that is used to add a bit of inner margin (0.1 by default)

When it's used horizontally (not the case for cartoframes) there's some extra logic to reduce the required space for the SVG.

On the vertical rendering, this doesn't happen. Partly because we can use the space to render long numbers without worrying too much.

[rjimenezda]

Now there's also as-legend-size-category which is a metacomponent for:

• as-legend-size-bins-point, which now supports marker as well.
• The new as-legend-size-category-line

[rjimenezda]

I've made elena's suggestions.

I'm going to keep this open to merge the specs branch into this one later.