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-parser/src/parser.rs

@@ -17,7 +17,7 @@ use nu_protocol::{
     engine::StateWorkingSet,
     eval_const::{eval_constant, value_as_string},
     span, BlockId, DidYouMean, Flag, ParseError, PositionalArg, Signature, Span, Spanned,
-    SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
+    SyntaxShape, Type, Unit, VarId, IN_VARIABLE_ID, NOTHING_VARIABLE_ID,
 };
 
 use crate::parse_keywords::{
@@ -1810,7 +1810,7 @@ pub fn parse_variable_expr(working_set: &mut StateWorkingSet, span: Span) -> Exp
 
     if contents == b"$nothing" {
         return Expression {
-            expr: Expr::Nothing,
+            expr: Expr::Var(nu_protocol::NOTHING_VARIABLE_ID),
             span,
             ty: Type::Nothing,
             custom_completion: None,
@@ -6118,7 +6118,9 @@ pub fn discover_captures_in_expr(
             discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
         }
         Expr::Var(var_id) => {
-            if (*var_id > ENV_VARIABLE_ID || *var_id == IN_VARIABLE_ID) && !seen.contains(var_id) {
+            if (*var_id > NOTHING_VARIABLE_ID || *var_id == IN_VARIABLE_ID)
+                && !seen.contains(var_id)
+            {
                 output.push((*var_id, expr.span));
             }
         }