"Reachable" effect

[lmoureaux]

Is your feature request related to a problem? Please describe.
In general, only units that are "reachable" can be attacked (with exceptions controlled by targets=). Which units are reachable is controlled by the Unreachable unit class flag and a few hard-coded rules: units in cities and bases are always reachable. This isn't very flexible: for instance, one may want submarines to be unreachable when in deep water.

Describe the solution you'd like
Add a Reachable effect that controls reachability of a unit based on the tile it's on (and maybe the action performed by the opponent). Unhardcode existing rules.

Describe alternatives you've considered
Can't think of any.

Additional context
@ec429 would like this for Avi.

[XHawk87]

Allowing testing the action performed could be useful for situations like this:

Example 1: You have a Frigate capable of both Bombard and Attack actions depending on the context. On the land is a Musketeer. The Frigate should be allowed to Bombard the Musketeer with its cannons but it can't directly Attack it because the Musketeer cannot fire back over the distance or close the distance.

Example 2: The same Frigate, but the land tile contains both a Musketeer and a Cannon. The Frigate should now only be allowed to Attack the Cannon, but NOT the Musketeer and it should NOT be allowed to Bombard because if it tried the Cannon would return fire.