src: add `--env-file-if-exists` flag

[BoscoDomingo]

Add a new flag, --env-file-if-exists that allows loading .env files without throwing an error if they don't exist.

Achieved by returning an env_file_data struct instead of just the paths of the .env files. This tells the loader if the file itself is required or not, thus allowing to not throw an error if the file doesn't exist and is optional, without breaking existing behaviours with --env-file (we could maybe show a warning if an optional file is missing?).

Usage:

node --env-file A.env --env-file-if-exists B.env

If B.env exists, it will load its environment variables and overwrite those that conflict with A.env. If it doesn't, node will continue as normal.

Fixes: https://github.com/nodejs/node/issues/50993 Refs: https://github.com/nodejs/node/issues/51451