Implements #379.

I decided to change the return type of the method to:

pub fn get_substitution_key(&self) -> Result<Vec<&str>, &'static str>

because of cases like FOO=$BAR$BAZ -> ["BAR", "BAZ"]
and because there could be an errors while parsing keys.

UPD: final function signature is:

pub fn get_substitution_key(&self) -> Vec<&str>

Function should stop parsing on error and return parsed keys: FOO=$BAR ${BAZ $BAM -> ["BAR"]

✔ Checklist:

• Commit messages have been written in Conventional Commits format;
• This PR has been added to CHANGELOG.md (at the top of the list);
• Tests for the changes have been added (for bug fixes / features);
• Docs have been added / updated on the dotenv-linter.github.io (for bug fixes / features).