First pass effort.

[jonathanpallant]

Adds a basic Rust application which spawns two threads.

Debugging in VS Code works, and cargo run works.

Demo app is designed to run on the nRF52840-DK.

[jonathanpallant]

TODO: Add Github CI, for:

• Building ThreadX and the Rust binary
• Checking Rust code formatting
• Checking the Reuse lint
• Running clippy

Edit: Done :)