feat: add ruler to measure distances

[maurerle]

Description

this adds a ruler to measure distances between routers:
@T-X

fixes #80

Motivation and Context

How Has This Been Tested?

Clicking around in dev mode

Screenshots/links:

Checklist:

• My code follows the code style of this project. (CI will test it anyway and also needs approval)
• My change requires a change to the documentation.
  • I have updated the documentation accordingly.

[T-X]

Yaiy, thanks a lot @maurerle, I'm loving it! Works great and will be super useful.

A few minor style things I noticed (and not sure if they could be enhanced universally/easily, I'd be happy if this gets merged as is, too):

The meter text is sometimes a bit hard to read, especially if there are multiple other lines and/or if that text is above its own dashed, black line:

.oO(Maybe a white outline around the black meter text could be added?)

In dark-mode on a satellite map the meter text is nicely visible in white, but instead the dashed line is hard to see:

.oO(Maybe a white outline around the dashed lines could be added?)

Link to our meshviewer test setup: https://map.luebeck.freifunk.net/meshviewer/#/en/map/18a6f7ca96ae

But my CSS knowledge is pretty much non-existent, if this is tricky to change then I'm really happy with this PR as is :-).