Add ability to set anchors/offsets for lines and pathtexts

[pastk]

It'll be helpful for many cases, e.g.

• render segregated shared paths for cyclists (a cycling line) and pedestrians (white dashed line) Convert between footways, paths, cycleways depending on surface, etc. #7878 (comment) or trails + ski pistes
• render area borders, e.g. for nature reserves, with visible inner and outer sides Native/Indian/Indigenous land is "unusable" (boundary=aboriginal_lands) #7812 (comment)
• one-sided barriers, etc.

Similar for pathtexts, we'd want to render area's name on the inner side of the border.

• also for Misoriented altitude labels on contour lines #7464

Basically its enough to have an anchor: left/right attrib. If its present the renderer should offset the line so that it runs exactly to the left/right of the "main" line (if present) or the default centerline.
If we add an ability to set both an anchor and an offset value, then it'd be possible to render slightly separated lines or partially overlapping ones, e.g.

Fancy steps? :) (two offset short dashed thick lines) But maybe we don't need this flexibility.

FYI @vng

[pastk]

It might be tricky for thicker lines though (e.g. for "fading" borders)
The turns might not look good unless specially processed

[pastk]

If we add an ability to set both an anchor and an offset value, then it'd be possible to render slightly separated lines or partially overlapping ones, e.g.

We'll need it to display cycleways over left or right sides of roads.