clangd config for ide integration

[relzp]

adds files (compile_flags.txt, .clangd) to configure clangd so that the clangd extension can be used in vscode (or anywhere with clangd support) for code highlighting, code completion, etc.
the config also does the following:

1. suppresses warnings about unused includes, since:

• we use includes like module.c -> module.h, where module.h includes the symbols that both the header and source need
• the stm32 library also uses includes like this, for example we define the modules we want and then include stm32h7xx_hal.h, instead of each individual module (e.g stm32h7xx_hal_spi.h)

2. suppresses errors from the stm32 libraries (under Drivers/)

currently, it uses the default compiler installed on the user's machine, not the target architecture (arm) compiler, for simplicity and so that each user doesn't have to install the cross-compiler