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-engine/src/eval.rs

@@ -6,8 +6,8 @@ use nu_protocol::{
         Expression, Math, Operator, PathMember, PipelineElement, Redirection,
     },
     engine::{EngineState, Stack},
-    IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Range, Record, ShellError, Span,
-    Spanned, Unit, Value, VarId, ENV_VARIABLE_ID,
+    report_error_new, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Range, Record,
+    ShellError, Span, Spanned, Unit, Value, VarId, ENV_VARIABLE_ID,
 };
 use std::collections::HashMap;
 
@@ -1127,6 +1127,20 @@ pub fn eval_variable(
     span: Span,
 ) -> Result<Value, ShellError> {
     match var_id {
+        // $nothing
+        nu_protocol::NOTHING_VARIABLE_ID => {
+            report_error_new(
+                engine_state,
+                &ShellError::GenericError(
+                    "Deprecated variable".into(),
+                    "`$nothing` is deprecated and will be removed in 0.87.".into(),
+                    Some(span),
+                    Some("Use `null` instead".into()),
+                    vec![],
+                ),
+            );
+            Ok(Value::nothing(span))
+        }
         // $nu
         nu_protocol::NU_VARIABLE_ID => {
             if let Some(val) = engine_state.get_constant(var_id) {