Inter-Job direct messaging (#15253)

# Description

This PR implements an experimental inter-job communication model,
through direct message passing, aka "mail"ing or "dm"ing:



- `job send <id>`: Sends a message the job with the given id, the root
job has id 0. Messages are stored in the recipient's "mailbox"
- `job recv`: Returns a stored message, blocks if the mailbox is empty
- `job flush`: Clear all messages from mailbox

Additionally, messages can be sent with a numeric tag, which can then be
filtered with `mail recv --tag`.
This is useful for spawning jobs and receiving messages specifically
from those jobs.

This PR is mostly a proof of concept for how inter-job communication
could look like, so people can provide feedback and suggestions

Closes  #15199

May close #15220 since now jobs can access their own id.

# User-Facing Changes

Adds, `job id`, `job send`, `job recv` and `job flush`  commands.

# Tests + Formatting

[X] TODO:  Implement tests
[X] Consider rewriting some of the job-related tests to use this, to
make them a bit less fragile.

# After Submitting

crates/nu-protocol/src/errors/shell_error/mod.rs

@@ -1370,7 +1370,7 @@ On Windows, this would be %USERPROFILE%\AppData\Roaming"#
 
     #[error("Job {id} is not frozen")]
     #[diagnostic(
-        code(nu::shell::os_disabled),
+        code(nu::shell::job_not_frozen),
         help("You tried to unfreeze a job which is not frozen")
     )]
     JobNotFrozen {
@@ -1379,6 +1379,27 @@ On Windows, this would be %USERPROFILE%\AppData\Roaming"#
         span: Span,
     },
 
+    #[error("The job {id} is frozen")]
+    #[diagnostic(
+        code(nu::shell::job_is_frozen),
+        help("This operation cannot be performed because the job is frozen")
+    )]
+    JobIsFrozen {
+        id: usize,
+        #[label = "This job is frozen"]
+        span: Span,
+    },
+
+    #[error("No message was received in the requested time interval")]
+    #[diagnostic(
+        code(nu::shell::recv_timeout),
+        help("No message arrived within the specified time limit")
+    )]
+    RecvTimeout {
+        #[label = "timeout"]
+        span: Span,
+    },
+
     #[error(transparent)]
     #[diagnostic(transparent)]
     ChainedError(ChainedError),