Haxe.runtime.FieldHost

[Simn]

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Opening a PR so I don't forget about the branch.

Originally, I wanted to only look into #11871, but then one thing led to another and I wanted to try and make our reflection API a bit more type-safe. To that end, we're adding haxe.runtime.FieldHost which can be assigned to from various things.

This might break some code, but if possible I'd like to get this done.

There are currently failures related to haxelib that I haven't looked at yet:

Development directory set to /home/runner/work/haxe/haxe/tests/misc/projects/Issue10844/dummy_doc_dep
Failed to set development directory to ../misc/projects/Issue10844/dummy_doc: * is not a valid library version