Function plan_introduce_variable 

pub fn plan_introduce_variable(
    file: &Path,
    content: &str,
    range: ByteRange,
    name: &str,
) -> Result<IntroduceVariableOutcome, String>

Build an introduce-variable plan without touching the filesystem.

file is the absolute path to the file. content is the current file content. range is the byte range of the expression to extract. name is the variable name to introduce.