[FEATURE] Add named constructor to `Marker`s to allow absolute positioning

[JaffaKetchup]

What do you want implemented?

Since v6, Markers can only be aligned relatively (on a scale of 0-1) through Alignment. It would be good for there to be an alternative constructor that takes a number of pixels from each edge, and the size of the Marker, and calculates the relative Alignment.

What other alternatives are available?

As an example, given the width (w) and number of pixels to offset from the left (d), the formula 1.0 - 2 * d / w can be used, as suggested by @Zverik in https://github.com/fleaflet/flutter_map/pull/1659#issuecomment-1849013832.

Can you provide any other information?

No response

Severity

Annoying: Currently have to use workarounds