Following are the extensions required for neovim:

• mason
• mason-lspconfig
• lspconfig

I use lazy.nvim so, just add the following to your plugins table, or create a new file in lua/plugins/lsp.lua

return {
	{
		"williamboman/mason.nvim",
		config = function()
			require("mason").setup({})
		end,
	},
	{
		"williamboman/mason-lspconfig.nvim",
		config = function()
			require("mason-lspconfig").setup({
				ensure_installed = { "lua_ls", "clangd" },
			})
		end,
	},
	{
		"neovim/nvim-lspconfig",
		config = function()
			local lspconfig = require("lspconfig")
			lspconfig.lua_ls.setup({})
            lspconfig.clangd.setup({})
		end,
	},
}

I've made a script to do all this for you:

$ cd path/to/the/project
$ curl https://gist.githubusercontent.com/ironlungx/a2b620de74f875c49c1d06999a8c41f8/raw/3f0d5d46c0d6369fc7e51e32487772ac194c27b1/script.sh | sh
$ pio run -t compiledb

BUT I DONT LIKE curl|sh:

1. Initialize a project with pio init

2. In the project root create the following files

   • .clangd:

     CompileFlags:                    
       Add:
         [
           # -mlong-calls,
           -DSSIZE_MAX,
           -DLWIP_NO_UNISTD_H=1,
           -Dssize_t=long,
           -D_SSIZE_T_DECLARED,
         ]
       Remove:
         [
           -fno-tree-switch-conversion,
           -mtext-section-literals,
           -mlongcalls,
           -fstrict-volatile-bitfields,
           -free,
           -fipa-pta,
         ]

   • .clang-tidy:

     Checks: '-*, -misc-definitions-in-headers' 

   • gen_compile_commands.py:

     import os
     Import("env")
     
     # include toolchain paths
     env.Replace(COMPILATIONDB_INCLUDE_TOOLCHAIN=True)
     
     # override compilation DB path
     env.Replace(COMPILATIONDB_PATH="compile_commands.json")

3. Now add the following to your platformio.ini

   extra_scripts = pre:gen_compile_commands.py

4. Now in your terminal run: pio run -t compiledb (Just delete the old compile_commands.json before executing this)

Hopefully this helped, I spent a lot of time figuring it out