Add configuration parameter for changing the target directory.

[jonhoo]

rls currently only allows changing the target build directory through the CARGO_TARGET_DIR environment variable. This is a little unfortunate given that some clients only support sending configuration options, and not other things like changing environment variables. See for example this workaround for LangaugeClient-neovim.

It'd be great if direct support could be added for a target_dir parameter, which changes what target directory is used, and which overrides (or perhaps, is relative to?) CARGO_TARGET_DIR. This is especially important given that cargo and rls step on each other's toes if they operate in the same directory (#866)

[Xanewok]

It is currently supported by the LSP and the RLS already has this parameter. Language clients need to use the LSP workspace/didChangeConfiguration to pass the rust.target_dir key (See #866 (comment) for more info), so I'm going to close the issue, unless it's not covered by that.

[jonhoo]

Ah, this wasn't listed under Configuration in the README. Perhaps that list should be updated accordingly?

Also, is the parameter named rust.target_dir or just target_dir? My understanding was that the rust. prefix is a VSCode-ism?

Separately, I've tried giving target_dir when CARGO_TARGET_DIR is set, and it seems as though the latter is still used. Are you sure that target_dir overrides CARGO_TARGET_DIR?