preserve variable capture spans in blocks (#15334)

Closes #15160

# User-Facing Changes

Certain "variable not found" errors no longer highlight the surrounding
block.

Before:

```nushell
do {
  match foo {
    _ => $in
  }
}

Error: nu:🐚:variable_not_found

  × Variable not found
   ╭─[entry #1:1:1]
 1 │ ╭─▶ do {
 2 │ │     match foo {
 3 │ │       _ => $in
 4 │ │     }
 5 │ ├─▶ }
   · ╰──── variable not found
```

After:

```nushell
Error: nu:🐚:variable_not_found

  × Variable not found
   ╭─[entry #1:3:10]
 2 │   match foo {
 3 │     _ => $in
   ·          ─┬─
   ·           ╰── variable not found
```

crates/nu-protocol/src/ast/block.rs

@@ -6,7 +6,7 @@ use serde::{Deserialize, Serialize};
 pub struct Block {
     pub signature: Box<Signature>,
     pub pipelines: Vec<Pipeline>,
-    pub captures: Vec<VarId>,
+    pub captures: Vec<(VarId, Span)>,
     pub redirect_env: bool,
     /// The block compiled to IR instructions. Not available for subexpressions.
     pub ir_block: Option<IrBlock>,

crates/nu-protocol/src/ast/expression.rs

@@ -111,7 +111,10 @@ impl Expression {
             Expr::UnaryNot(expr) => expr.has_in_variable(working_set),
             Expr::Block(block_id) | Expr::Closure(block_id) => {
                 let block = working_set.get_block(*block_id);
-                block.captures.contains(&IN_VARIABLE_ID)
+                block
+                    .captures
+                    .iter()
+                    .any(|(var_id, _)| var_id == &IN_VARIABLE_ID)
                     || block
                         .pipelines
                         .iter()

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

@@ -322,12 +322,12 @@ impl Stack {
         }
     }
 
-    pub fn gather_captures(&self, engine_state: &EngineState, captures: &[VarId]) -> Stack {
+    pub fn gather_captures(&self, engine_state: &EngineState, captures: &[(VarId, Span)]) -> Stack {
         let mut vars = Vec::with_capacity(captures.len());
 
         let fake_span = Span::new(0, 0);
 
-        for capture in captures {
+        for (capture, _) in captures {
             // Note: this assumes we have calculated captures correctly and that commands
             // that take in a var decl will manually set this into scope when running the blocks
             if let Ok(value) = self.get_var(*capture, fake_span) {