toolchain: v38; clangd as default language server (#3774)

* libs: removed cxxheaderparser submodule, expecting one from toolchain
* toolchain: v38
* vscode: now using clangd from toolchain
* vscode: clangd path in config is now generated by fbt
* vscode, fbt: improved clangd path generation
* fbt: fixed LANG_SERVER handling; switched to clangd as default
* vscode: removed deprecated options from config
* ufbt: project template: updated clang-format, added clangd config
* ufbt: now using clangd as default language server
* ufbt: now using clangd as default language server

scripts/ufbt/project_template/.vscode/extensions.json

@@ -8,7 +8,7 @@
 	// List of extensions which should be recommended for users of this workspace.
 	"recommendations": [
 		"ms-python.black-formatter",
-		"ms-vscode.cpptools",
+		"llvm-vs-code-extensions.vscode-clangd",
 		"amiralizadeh9480.cpp-helper",
 		"marus25.cortex-debug",
 		"zxh404.vscode-proto3",
@@ -18,6 +18,7 @@
 	// List of extensions recommended by VS Code that should not be recommended for users of this workspace.
 	"unwantedRecommendations": [
 		"twxs.cmake",
+		"ms-vscode.cpptools",
 		"ms-vscode.cmake-tools"
 	]
 }