Improve vscode clangd experience (#2431)

* Improve vscode clangd experience

- Resolve and use absolute path for the toolchain. This allows clangd to use compile_commands.json file without running under fbtenv, simplifies setup for vscode clangd extension. As a side effect, a rebuild is needed to update compile_commands.json after moving the source tree.
- Add the recommended default settings of the extension to settings.json.

* Use build/latest for compile-commands-dir

This makes it behave closer to c-cpp-properties.

* Reformat crosscc.py

This is a PEP-8 violation but black seems to enforce it

* Bypass --query-driver

This has some security implications as it grants clangd the ability to execute any executables anywhere while trying to probe a compiler based on CDB. However it's very hard to do this the safe and intended way without resorting to config generation due to reason listed in #2431. Besides that we already have workspace trust so what could go wrong? ;)

* Add an option for vscode_dist to switch between clangd and cpptools

This will install different extensions.json tuned for either clangd or cpptools based on user selection. It will also install c_cpp_properties.json when using cpptools since clangd doesn't use this file.

The root .gitignore now also doesn't accidentally ignore everything under the .vscode directory.

* Use absolute path for .vscode gitignore

Turns out the previously used "relative" paths aren't even valid gitignore patterns and to actually do what it means one needs to use the absolute paths instead.

* Handle variable parsing in commandline.scons

commandline.scons is the place where all other command line parsing happens. Move LANG_SERVER variable parsing there and add a constraint to make the code more consistent.

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Co-authored-by: hedger <hedger@users.noreply.github.com>
Co-authored-by: hedger <hedger@nanode.su>

documentation/fbt.md

@@ -46,6 +46,8 @@ To run cleanup (think of `make clean`) for specified targets, add the `-c` optio
 
 `fbt` includes basic development environment configuration for VS Code. Run `./fbt vscode_dist` to deploy it. That will copy the initial environment configuration to the `.vscode` folder. After that, you can use that configuration by starting VS Code and choosing the firmware root folder in the "File > Open Folder" menu.
 
+To use language servers other than the default VS Code C/C++ language server, use `./fbt vscode_dist LANG_SERVER=<language-server>` instead. Currently `fbt` supports the default language server (`cpptools`) and `clangd`.
+
 - On the first start, you'll be prompted to install recommended plugins. We highly recommend installing them for the best development experience. _You can find a list of them in `.vscode/extensions.json`._
 - Basic build tasks are invoked in the Ctrl+Shift+B menu.
 - Debugging requires a supported probe. That includes: