Chore/rust scripts to rule them all

[snim2]

Remove PHP code and add scripts-to-rule-them-all for Rust.

Description

This PR sets up the repo for Rust development. It:

• Removes all PHP code and tooling
• Adds Rust tooling and scripts in the scripts-to-rule-them-all pattern
• Adds a hello-world app automatically generated by cargo init
• Adds a unit test for that app to ensure that the test scripts work
• Adds a GitHub action to run the test scripts
• Adds architecture decision records which now includes the decision to try Rust, experimentally

This PR does not:

• Attempt to do anything related to Whippet features
• Attempt to build a "release" of the Whippet binary or produce any other artefacts

Note that this PR merges into a new protected branch, experiment/whippet-3.0 because we may choose not to keep this code.

Testing

1. Checkout this branch
2. Run ./script/test if I've got the Brew dependencies right then you should see something like:

❯ ./script/test
==> Building...
==> Updating...
==> Bootstrapping...
==> Updating Cargo dependencies...
     Locking 0 packages to latest Rust 1.86.0 compatible versions
==> Cleaning old files...
     Removed 0 files
==> Building Whippet...
   Compiling whippet v3.0.0 (.../whippet)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.42s
==> Running ShellCheck...
Checking ./script/bootstrap
Checking ./script/test
Checking ./script/update
Checking ./script/setup
Checking ./script/build
Checking ./.shellcheck.sh
OK
==> Running code formatter...
==> Running code linter...
    Checking whippet v3.0.0 (/Users/sarah.mount/code/github.com/dxw/whippet)
    Finished `dev` profile [unoptimized + debuginfo] target(s) in 0.12s
==> Running the tests...
   Compiling whippet v3.0.0 (/Users/sarah.mount/code/github.com/dxw/whippet)
    Finished `test` profile [unoptimized + debuginfo] target(s) in 0.31s
     Running unittests src/main.rs (target/debug/deps/whippet-5e4893a47ad8b6ef)

running 0 tests

test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s

     Running tests/hello_world.rs (target/debug/deps/hello_world-8940bbc8d2d2757d)

running 2 tests
test test_correct_output ... ok
test test_application_runs ... ok

test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.30s

• Includes tests (if new features are introduced)
• Commit message includes link to the ticket/issue
• Changelog updated
• PR open against dxw's Homebrew Tap to point the Whippet formula to the new version number

[RobjS]

@snim2 Currently I'm getting this from script/test:

...
==> Running ShellCheck...
Checking ./script/bootstrap
Checking ./script/test
Checking ./script/update
Checking ./script/setup
Checking ./script/build
Checking ./.shellcheck.sh
OK
==> Running code formatter...
Invalid value for `--edition`

[snim2]

@snim2 Currently I'm getting this from script/test:

...
==> Running ShellCheck...
Checking ./script/bootstrap
Checking ./script/test
Checking ./script/update
Checking ./script/setup
Checking ./script/build
Checking ./.shellcheck.sh
OK
==> Running code formatter...
Invalid value for `--edition`

OK, what do you get as the output of these commands?

rustc --version
rustfmt --version
cargo --version