Add SDL3 microui

[greduan]

Note this is an adapted copy of @BemusedButBloodthirsty 's earlier work #109 (not sure why it didn't get merged or why the PR got closed).

Works on my machine right now (macOS):

><> odin build program -vet -strict-style -vet-tabs -disallow-do -warnings-as-errors
><> odin run program -vet -strict-style -vet-tabs -disallow-do -warnings-as-errors

Checklist before submitting:

• This example has been added to .github/workflows/check.yml (for automatic testing)
• This example compiles cleanly with flags -vet -strict-style -vet-tabs -disallow-do -warnings-as-errors
• This example the naming and style convention: https://github.com/odin-lang/examples/wiki/Naming-and-style-convention (exceptions can be made for ports of examples that need to match 1:1 to the original source).
• By submitting, I understand that this example is made available under these licenses: Public Domain and Odin's zlib license. Only for third-party dependencies are other licenses allowed.