db.pg: Support returning Result type with column names

[omerrob]

Currently, db.pg queries return []Row, which is just

pub struct Row {
pub mut:
	vals []?string
}

Then the developer needs to map the values based on ordinal places.

I understand that this was built with the ORM in mind, and the ORM maintains a mapping of the ordinal places with the struct members.

Sometimes we don't want to use the ORM or can't because we use a lot of joins and CTEs (WITH clauses). In that case, it is convenient to have the column names in order to map the query.

This PR adds the Result struct:

pub struct Result {
pub:
	cols map[string]int
	rows []Row
}

and adds matching fn *_result() functions to the currently implemented functions.

It also adds a quality-of-life mapping method to Result:

fn (res Result) as_structs[T](mapper fn (Result, Row) !T) ![]T

that accepts a mapper function to return a typed struct.

[spytheman]

https://github.com/vlang/v/actions/runs/19352895759/job/55368240782?pr=25737 shows that the PR needs the new pub APIs to be documented.
Please add a small comment on top of each, starting with the function/method name.

[omerrob]

Added function documentation.

[JalonSolov]

None of those "Unhandled Exception" codes are valid codes.

[omerrob]

Sorry, not sure what you mean by "Unhandled Exception" codes.

[JalonSolov]

Not you - in the CI failure logs.

[spytheman]

The CI is a bit flaky unfortunately in the last few days :-| .

[JalonSolov]

Merge master again. Should take care of the Windows failures, at least.

[spytheman]

Excellent work.

[spytheman]

Thank you @omerrob 🙇🏻 .