Add a new RFC for Meta Spec v2. Based on the [PGXN Meta Sketch] and
additional research. Notable differences from v1:

*   Introduce the term "package" separate from "distribution". The
    "package" is the bundle of objects being distributed as a…package.
*   Use only one version for the entire distribution, rather than
    separate versions for each extension included in the distribution.
    This allows the deletion of the notes on comparing versions.
*   Make "Source Distribution" the formal term, but mostly refer to it
    as "Distribution". Will be distinguished from binary distributions
    that ship with their own metadata (see #2).
*   Use JSON data types as the base types instead of generic "list" and
    "map" types.
*   Add "Path", "purl", and "Platform" types
*   Use SPDX License Expressions instead of Perl Software::License-based
    structures.
*   Use the term "property" to describe object key/value pairs, to align
    with JSON Schema.
*   Use an array of objects to describe maintainers.
*   Replace the `provides` property with `contents`, with support for
    multiple kinds of PostgreSQL extensions, including TLEs, loadable
    modules, and background workers.
*   Move the `tags` property to the new `classifications` object, and
    add support for curated categories borrowed from [Trunk].
*   Replace `no_index` with `ignore` and use the gitignore format
    instead of separate lists of files and directories.
*   Rename `prereqs` to `packages` and move it into the new
    `dependencies` property, which also has `postgres`, `pipeline`,
    `platforms`, and `variations` properties. Use [purls]. to specify
    dependencies, so that any supported packaging dependency can be
    specified, as well as PGXN packages and Postgres core extensions and
    tools.
*   Remove `release_status`; we'll instead depend on [SemVer] to
    indicate pre-releases.
*   Simplify the `resources` object and add `badges` to it.
*   Add the `artifacts` property, so the extension author can include
    links to other packages or sources for a release.

Also configure `#` to hide a line in `json` code blocks and use it to
encode proper JSON objects without showing the surrounding braces.
Readers can hit the eye button that appears on hover to make the hidden
lines appear.

[PGXN Meta Sketch]: https://justatheory.com/2024/03/rfc-pgxn-metadata-sketch/
[Trunk]: https://pgt.dev
[purls]: https://github.com/package-url/purl-spec/blob/master/PURL-SPECIFICATION.rs
[SemVer] https://semver.org

*   Add the "Package Name" type that disallows leading digits.
    [Discussion](#2 (comment)).
*   Document the Number type and allow `0` as a valid value for a
    Version Range.
*   Add the "Path Pattern" type and use it for the "ignore" property.
*   Merge "License" into "License Expression"
*   Rename "Version" to "SemVer", to distinguish it from any other
    versions used for dependencies.
*   Make "Version Range" not-specific to SemVers, since it's used for
    all sorts of dependency version requirements. Also, disallow version
    truncation in Version Ranges, since different version formats will
    have different rules.
*   Document that purl version expressions are valid but ignored.
*   Fix various spelling, grammatical, syntax, and narrative errors and
    clumsiness.
*   Update spec URLs to point to rfcs.pgxn.org.
*   Rename `generated_by` to `producer`.
*   Add question about preloading.
*   Note quality binary distribution as sign of success.

Instead of having a separate "package name" type. A quick check of the
PGXN database and [1000+ PostgreSQL EXTENSIONs] shows no existing
extensions contain a dot. This will simplify parsing a package name from
a semver in file names, since dots are required for semantic versions.

  [1000+ PostgreSQL EXTENSIONs]: https://gist.github.com/joelonsql/e5aa27f8cc9bd22b8999b7de8aee9d47

* extensions: `CREATE EXTENSION` extensions
* modules: loadable modules for extensions, hooks, and workers
* apps: Programs and scripts like pg_top, pgAdmin, or pg_partman scripts

Add an example from pg_partman, which includes all three types.

*   Add missing `contents` key to example
*   Rename "Path Pattern" to "Glob"
*   Fix ignore header
*   Fix missing package version
*   Use "cargo" instead of "rust" for pipeline value
*   Tweak wording on phases
*   Add url property to badge

Also link to the Repology API.

And fix a couple of version references and links.