Feature: edit server path and options in preference page

[ghentschke]

The server path and options can be edited for the used server provider. The default values are the commands delivered from the server provider.

In Windows it looks like this:

TODO: add tests and create a new issue for the topic that the LS has to be restarted when the server binary or options changes

[jonahgraham]

This change points out a problem that we'll need to resolve at some future point. Different projects in the workspace may need different clangd installs. I don't think we should put this preference as a project setting as well as that would add hard coded paths to the project settings.

I think we can defer it for now, but it is something to keep in mind. This becomes of special interest to Renesas IIUC because they have special versions of clang(d) for different processors.

[ghentschke]

Maybe then it's a better approach to let C/C++ toolchain(s) provide the server path and settings. (That's what we do in our projects). The toolchain plugin could use the serverProvider extension point from org.eclipse.cdt.lsp to provide a server. Instead of using priorities to select a distinct server, we can use the isEnabledFor method in the org.eclipse.cdt.lsp.server.ICLanguageServerProvider interface to select a server provider for a project.

This leads to new LSP4E requirement: Run a language server instance per project.
This will solve another issue:
Currently the compile settings (examined from the found compile_commands.json) from the first opened project file which triggers the LS will be used workspace wide to examine (e.g. the C/C++ version to use) the included external (header) files.

Instead, the project specific compile settings should be used to examine external (#include) files included in a project file (e.g. a C++20 system header is included in a project where the C++20 standard has been defined). Another project in the workspace could use a different standard ans system library.

I opened a new issue to discuss this requirement further: #22

[jonahgraham]

LGTM now.