Add parseFlakeRef and flakeRefToString builtins (#8670)

Over the last year or so I've run into several use cases where I need to
parse and/or serialize URLs for use by `builtins.fetchTree` or
`builtins.getFlake`, largely in order to produce _lockfile-like_ files
for lang2nix frameworks or tools which use `nix` internally to drive
builds.

I've gone through the painstaking process of emulating
`nix::FlakeRef::fromAttrs` and `nix::parseFlakeRef` several times with
mixed success; but these are difficult to create and even harder to
maintain if I hope to stay aligned with changes to the real
parser/serializer.

I understand why adding new `builtins` isn't something we want to do
flagrantly. I'm recommending this addition simply because I keep
encountering use cases where I need to parse/serialize these URIs in
`nix` expressions, and I want a reliable solution.

Co-authored-by: Eelco Dolstra <edolstra@gmail.com>
Co-authored-by: John Ericson <git@JohnEricson.me>

doc/manual/src/release-notes/rl-next.md

@@ -1 +1,8 @@
 # Release X.Y (202?-??-??)
+
+- Two new builtin functions,
+  [`builtins.parseFlakeRef`](@docroot@/language/builtins.md#builtins-parseFlakeRef)
+  and
+  [`builtins.flakeRefToString`](@docroot@/language/builtins.md#builtins-flakeRefToString),
+  have been added.
+  These functions are useful for converting between flake references encoded as attribute sets and URLs.