Add basic command task templating

[LinuCC]

• Parse the command.cmd JSON string as a CommandTask with CommandTaskVariables instead of a simple String
• Add rendering method replacing those variables (with some kind of default, e.g. empty string)
• Add form to input all the template variables

[flodiebold]

du kannst als replacer eine FnMut(&Captures) -> String angeben, d.h. du könntest alle variablen auf einmal abhandeln (und die gleiche regex wie weiter oben verwenden)

[LinuCC]

Well, in that case how would I return the error from the closure?
E.g.

    pub fn to_executable_string(<···>) -> Result<String, Box<std::error::Error>> {
        let template_regex = Regex::new(r"\{\{(.+?)\}\}").unwrap();
        output = template_regex
            .replace_all(&output, |caps: &Captures| {
                variables.get(&caps[0]).ok_or(CommandTaskReplaceValuesError)? // <- Error
            })
            .to_string();
    // <···>

Does not compile because the error now gets returned inside the closure/.

[flodiebold]

Yeah, you can't... you could first check that all the variables have values, and then do the actual replacing, though

[flodiebold]

is it actually possible that variables are missing from the hash map though?