Use errors.Join when available

Introduce HTTPError

This lets the caller inspect the HTTP response error if any.

Upgrade dependencies

gqlclientgen: Fix comment that indicates that code is generated

The period is missing at the end for the regex to match correctly.
See: https://godocs.io/cmd/go#hdr-Generate_Go_files_by_processing_source

cmd/gqlclientgen: avoid infinite loop when unmarshalling interface/union

cmd/gqlclientgen: document Value field for GQL interfaces/unions

cmd/gqlclientgen: generate Go interfaces for each GQL union/interface

cmd/gqlclientgen: fix unmarshalling of base interface fields

gqlclientgen: unmarshal underlying type for interfaces

Upgrade dependencies

gqlclient: introduce a Time type

A small struct wrapper around time.Time which correctly maps null
JSON values to the zero time. Additionally it uses the
time.RFC3339Nano format like gqlgen does.

I've considered using pointers but that'd force callers to
explicitly check for nil values instead of handling idiomatic zero
time values.

Upgrade dependencies

cmd/gqlclientgen: recursively collect fragments

This makes fragments that include other fragments work.

readme: fix typo

cmd/*: print usage to stderr

readme: switch to dedicated mailing list

readme: fix code block syntax

readme: mention gqlintrospect utility

cmd/gqlintrospect: add usage

cmd/gqlclientgen: add flag to omit deprecated fields

Add a new -d flag to omit deprecated fields and enum values.