WIP: allow setting metadata on zones

[KristofferC]

This will take some iterations and feedback to get right I think @topolarity.

First, the code in the test produces:

Some remarks:

• You probably want to be able to set the "default" color of a zone at a location (which is stored in the TracySrcLoc) as well as being able to set a "highlighting" color which is passed as a runtime call. That suggests we need two arguments for the color. So the question is what should they be called, color and dynamic_color?
• The same applies to the name. name and dynamic_name?
• The non-dynamic expressions need to be statically evaluable.

TODO:

• [ ] Update docs
• [ ] Decide on names (right now there is placeholder name2 as an example).

[KristofferC]

Thinking about https://github.com/topolarity/Tracy.jl/pull/19#discussion_r1199167779 I think this PR is misdesigned. Even for things like color or name which you can only set once for a zone, you might still want to set it based on some condition inside the zone. So none of these should probably go on the macro.