entities/x: Allow actual metadata usage

[Implausiblyfun]

Entities have a concept of metadata which is an unexported map.
This is a pretty cool feature and we've used it via unreleased branches before.

However since it is un-exported and uninitialized anyone who does not reimplement accessors on some wrapping class cannot use it.

In this PR we look to change that by allowing initialization of the map by default on entity creation.

To reproduce take an entity such as the one in platformer example https://github.com/oakmound/oak/blob/v4.1.0/examples/platformer/main.go#L28

char := entities.New(ctx, entities.WithRect(floatgeom.NewRect2WH(100, 100, 16, 32)), entities.WithColor(color.RGBA{255, 0, 0, 255}), entities.WithSpeed(floatgeom.Point2{3, 7}), )

then try to set some metadata
` char.SetMetadata("mykey", "somevalue")

`

[codecov-commenter]

⚠️ Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.98%. Comparing base (3b840e6) to head (99c5305).
Report is 2 commits behind head on master.

❗ Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #225   +/-   ##
=======================================
  Coverage   94.98%   94.98%           
=======================================
  Files         143      143           
  Lines        6403     6403           
=======================================
  Hits         6082     6082           
  Misses        288      288           
  Partials       33       33           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.