Support empty/default contexts, put chrono/regex behind features

[clarkmcc]

Closes #90

[Caellian]

Looks good.

Nitpick

You could remove guards on (and guarded) use satatements and use absolute paths for structs that are used in feature gated functions.

For instance:

#[cfg(feature = "regex")]
pub fn matches(
    ftx: &FunctionContext,
    This(this): This<Arc<String>>,
    regex: Arc<String>,
) -> Result<bool> {
    match regex::Regex::new(&regex) {
        Ok(re) => Ok(re.is_match(&this)),
        Err(err) => Err(ftx.error(format!("'{regex}' not a valid regex:\n{err}"))),
    }
}
// Allows you to remove:
#[cfg(feature = "chrono")]
use regex::Regex;

That's mostly a stylistic choice though - functionally, this is what I had in mind with #90.

[clarkmcc]

@Caellian love it, thanks for the feedback