Refactor compiler error codes

[pront]

The Problem

Currently we assign arbitrary error codes in different modules e.g.

    fn code(&self) -> usize {
        // ...
        match self {
            Undefined { .. } => 105,
            WrongNumberOfArgs { .. } => 106,
            UnknownKeyword { .. } => 108,
            Compilation { .. } => 610,
            // ...
        }
    }

These codes are scattered across different files and there's nothing preventing us from reusing them.

Solution

We should aggregate these under a single enum in src/compiler/errors.

Without giving this too much thought (might revisit later), we can have static arrays of codes:

const FUNCTION_CALL_ERROR_CODES: &[(FunctionCallError, usize)] = &[
    (Undefined, 105),
    (WrongNumberOfArgs, 106),
    (UnknownKeyword, 108),
    (Compilation, 610),
    // ...
];

and then use them like so:

impl DiagnosticMessage for FunctionCallError {
    fn code(&self) -> usize {
        use FunctionCallError::*;

        FUNCTION_CALL_ERROR_CODES
            .iter()
            .find(|&&(ref variant, _)| variant == self)
            .map(|&(_, code)| code)
            .expect("Error code not found for FunctionCallError variant")
    }
}

Docs

If we find duplicates and have to re-assign codes, we will also need to update:
https://vector.dev/docs/reference/vrl/errors/#compile-time-errors

Changing codes assigned to errors IMO isn't a breaking change.

To VRL users, if you use these codes, please comment in this ticket!

References

Ideally these should be generated from code, see #280.