Add support for developing with rust-analyzer

[nspin]

Before this PR, this project only supported developing with rust-analyzer in the ./hacking workspace, which only contains development tools. This PR adds the configuration, tools, and scripts necessary to enable leveraging rust-analyzer for development on the rest of the crates in this project.

This PR just adds support for rust-analyzer via VSCode, but the approach taken could be retargeted to other editor environments.

The challenge has always been that some of the crates in this project target the development system and require libstd, whereas others target seL4 userspace and require an active seL4 configuration. The solution in this PR is to add a script that wraps rust-analyzer, supplying it with some Cargo configuration (e.g. pointing to an active seL4 configuration) and limiting it to a subset of the crates in the project. .vscode/settings.json points VSCode to this wrapper.

When opening this project in VSCode, a default configuration is chosen, without any additional Cargo configuration, and limited to the subset of crates that can be compiled without a seL4 configuration.

The create-vscode-workspace tool can be used to generate a *.code-workspace file that, when opened in VSCode, will run rust-analyzer with the given config and crate subset. For example:

$ make cargo-config
$ cargo run \
    --manifest-path=hacking/Cargo.toml \
    -p create-vscode-workspace -- \
    --config .cargo/gen/target/aarch64-sel4-microkit.toml \
    --config .cargo/gen/world/aarch64.microkitDefault.toml \
    --include microkit-http-server-example-server \
    -o foo.code-workspace
$ code foo.code-workspace

Future PRs with stabilize, improve usability, add docs, and check in some workspace files.