Register Compiler subfolder of JuliaLang/julia (#118541)

* Pre-registration PR for new pseudo-stdlib `Compiler`

This is a companion PR to JuliaLang/julia#56409. The two PRs will need to be merged somewhat
contemporaneously to allow downstream consumers to transition their use of
`Core.Compiler` to the `Compiler` pseudo-stdlib without interruption.

As described in JuliaLang/julia#56409, the idea here is rather than using the tranditional
Julia stdlib mechanism, which does not track exact package contents or versions, the
Compiler instead becomes an ordinary Julia package that just happens to live in
JuliaLang/julia (for now). Base has a private copy of this package, but users
ordinarily aren't supposed to interact with it.

Since it is an ordinary package, it will need to be registered in General, rather than
relying on the entry it would have otherwise had in the julia distribution's `stdlib`
folder. This PR is intended to facilitate that registration.

Note that I am opening this PR without any actual tagged versions of `Compiler`.
My intention is to tag `0.0.1` upon merging the corresponding julia PR, but the
full versioning strategy for `Compiler` has yet to be decided upon.

As mentioned, the purpose of this PR is to get pre-agreement on the registration
itself to make sure we can merge/tag simultaneously without doing something
overly special on the registry side. I do not know if this PR can be merged as
is without any versions tagged or if some part of the tooling will be confused
by that. If it can, that's probably easiest. If not, I will amend this PR with
the appropriate version once the julia PR is merged and expect it to merge manually
shortly thereafter.

* Tag merged version as 0.0.1

* Correct syntax

C/Compiler/Package.toml

@@ -0,0 +1,4 @@
+name = "Compiler"
+uuid = "807dbc54-b67e-4c79-8afb-eafe4df6f2e1"
+repo = "https://github.com/JuliaLang/julia.git"
+subdir = "Compiler"

C/Compiler/Versions.toml

@@ -0,0 +1,2 @@
+["0.0.1"]
+git-tree-sha1 = "e65451228ef54619e15febc137138f2ff030eac3"

Registry.toml

@@ -5918,6 +5918,7 @@ some amount of consideration when choosing package names.
 80713f31-8817-5129-9cf8-209ff8fb23e1 = { name = "ImageSegmentation", path = "I/ImageSegmentation" }
 807425ed-42ea-44d6-a357-6771516d7b2c = { name = "RecurrenceRelationships", path = "R/RecurrenceRelationships" }
 807c15e4-954b-4072-b0d2-8db75ef7308a = { name = "EasyML", path = "E/EasyML" }
+807dbc54-b67e-4c79-8afb-eafe4df6f2e1 = { name = "Compiler", path = "C/Compiler" }
 807e6dab-69e6-5de1-b50c-dbd9513c50df = { name = "SerialDependence", path = "S/SerialDependence" }
 8083989d-5216-488c-bbe7-b79abd7e3507 = { name = "PointwiseKDEs", path = "P/PointwiseKDEs" }
 80847211-bc6c-46c0-860f-0841d8001362 = { name = "EyeOfRa", path = "E/EyeOfRa" }