remove the $nothing variable (#10478)

related to 
- https://github.com/nushell/nushell/pull/9973
- https://github.com/nushell/nushell/pull/9918

thanks to @jntrnr and their super useful tips on this PR, i learned
about the parser + evaluation, so 🙏

# Description
because we already have `null` as the value of the type `nothing` and as
a followup to the two other attempts of mine, i propose to remove the
redundant `$nothing` built-in variable 😋

this PR is the first step, deprecating `$nothing`.
a followup PR will remove it altogether and wait for 0.87 👍 

⚙️ **details**: a new `NOTHING_VARIABLE_ID = 3` has been added,
parsing `$nothing` will create it, finally a `Value::Nothing` will be
produced and a warning will be reported.

this PR already fixes the `toolkit.nu` module so that it does not throw
a bunch of warnings each time 👌

# User-Facing Changes
`$nothing` is now deprecated and will be removed in 0.87
```nushell
> $nothing
Error:   × Deprecated variable
   ╭─[entry #1:1:1]
 1 │ $nothing
   · ────┬───
   ·     ╰── `$nothing` is deprecated and will be removed in 0.87.
   ╰────
  help: Use `null` instead
```

# Tests + Formatting
tests have been updated, especially
- `nothing_fails_string`
- `nothing_fails_int`
which use a variable called `nil` now to make sure `nothing` does not
support cell paths 👍

# After Submitting
classic deprecation mention 👍

crates/nu-protocol/src/engine/engine_state.rs

@@ -113,6 +113,7 @@ const REGEX_CACHE_SIZE: usize = 100; // must be nonzero, otherwise will panic
 pub const NU_VARIABLE_ID: usize = 0;
 pub const IN_VARIABLE_ID: usize = 1;
 pub const ENV_VARIABLE_ID: usize = 2;
+pub const NOTHING_VARIABLE_ID: usize = 3;
 // NOTE: If you add more to this list, make sure to update the > checks based on the last in the list
 
 impl EngineState {

crates/nu-protocol/src/lib.rs

@@ -27,7 +27,7 @@ pub use alias::*;
 pub use cli_error::*;
 pub use config::*;
 pub use did_you_mean::did_you_mean;
-pub use engine::{ENV_VARIABLE_ID, IN_VARIABLE_ID, NU_VARIABLE_ID};
+pub use engine::{ENV_VARIABLE_ID, IN_VARIABLE_ID, NOTHING_VARIABLE_ID, NU_VARIABLE_ID};
 pub use example::*;
 pub use exportable::*;
 pub use id::*;