Add vendor:microui

This is a direct port in Odin, not bindings; utilizing Odin's rich type system

vendor/microui/README.md

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+# microui-odin
+
+## Description
+A tiny, portable, immediate-mode UI library written in Odin. (Ported from [rxi/microui](https://github.com/rxi/microui).)
+
+![screenshot](https://user-images.githubusercontent.com/3920290/56437823-c3dcdb80-62d8-11e9-978a-a0739f9e16f0.png)
+
+[**Browser Demo**](https://floooh.github.io/sokol-html5/sgl-microui-sapp.html) (rxi's microui)
+
+## Features
+* Tiny: around `1200 sloc` of Odin
+* Works within a fixed-sized memory region: no additional memory is
+  allocated
+* Built-in controls: window, panel, button, slider, textbox, label,
+  checkbox, wordwrapped text
+* Easy to add custom controls
+* Simple layout system
+
+## Notes
+* This library assumes you are using the latest nightly build or GitHub master of the Odin compiler. Since Odin is still under development this means this library might break in the future. Please create an issue or PR if that happens. Last verified against: odin version dev-2021-07:481fc8a5
+* The library expects the user to provide input and handle the resultant
+  drawing commands, it does not do any drawing itself.
+
+## License
+This library is free software; you can redistribute it and/or modify it
+under the terms of the MIT license. See [LICENSE](LICENSE) for details.