Add loading of packaged libraries by feature flag

[Xcodo]

From #45:

I refactored the configuration loading to make it easy to add loading of installed configuration: See: https://github.com/kraigher/rust_hdl/blob/master/vhdl_parser/src/config.rs#L236-L239
You only need to add a load_installed_config there to implement it.

I've implemented this function. I'm only up to chapter 5 in the Rust book, so improvements are welcome!

I've not added any tests. Am I right in thinking there's none for the other configuration loading functions either? I'm also couldn't work out how you add tests for whether a feature flag is enabled or not anyway.

I think we should use the cfg! macro or attribute to set the relative path to the installed vhdl_ls.toml file. By default it think the relative path should assume the binary ends up in target/release such that when building locally with cargo build --release during development it will just work without having to create the installed folder structure.

When building for release we can set a --cfg clag to rustc (https://doc.rust-lang.org/rustc/command-line-arguments.html#--cfg-configure-the-compilation-environment) such that the relative path is according to the install folder format.

I was a bit less successful here. From reading up on this the supported way of doing this with Cargo seems to be through feature flags. I implemented that for the vhdl_parser project with the packaged feature, which is also added to the vhdl_ls project to pass it through.

While this seems to be working for building it locally to the project

rust_hdl\vhdl_parser> cargo build --features "packaged"

It looks like feature flags aren't supported when building at the workspace level at the moment. The CI build could build from within each folder separately, but that's not what it does now.

[kraigher]

Looks good. Building releases from the workspace might not be necessary. The release can be built from the crate instead.

[Xcodo]

Looks like the way to do this with the cargo action is this. I'll implement that once @Bochlin has finished improving the CI setup.

[Xcodo]

Ready to go now I think @kraigher