Feature request: adding arduino-cli option to set warning level (--warnings <level>)

[da-phil]

See discussion #1508

The arduino-cli has a CLI option --warnings string Optional, can be: none, default, more, all. Used to tell gcc which warning level to use (-W flag). (default "none") to set which warning compiler flags are used during compilation.
As a reminder, those are:

        ["compiler.warning_flags",""],
        ["compiler.warning_flags.none", ""],
        ["compiler.warning_flags.all",""],
        ["compiler.warning_flags.extra",""]

In previous vscode-arduino versions somehow it was possible to set this option via buildPreferences:

"buildPreferences": [
        ["compiler.warning_level", "all"]
]

However this does not work anymore, as those are directly parsed to ``--build-property STRING` CLI options, see here:

vscode-arduino/src/arduino/arduino.ts

Line 151 in 6f24815

if (this.useArduinoCli()) {

Can we either also expose this CLI option to the vscode-arduino plug-in (clean approach) or add a special case for compiler.warning_level when parsing buildPreferences to set

["--warnings", `${key}=${value}`]);

which is not so clean and rather a "temporary" solution 😇 .

What do you think?