Improve type hovers (#9515)

# Description

This PR does a few things to help improve type hovers and, in the
process, fixes a few outstanding issues in the type system. Here's a
list of the changes:

* `for` now will try to infer the type of the iteration variable based
on the expression it's given. This fixes things like `for x in [1, 2, 3]
{ }` where `x` now properly gets the int type.
* Removed old input/output type fields from the signature, focuses on
the vec of signatures. Updated a bunch of dataframe commands that hadn't
moved over. This helps tie things together a bit better
* Fixed inference of types from subexpressions to use the last
expression in the block
* Fixed handling of explicit types in `let` and `mut` calls, so we now
respect that as the authoritative type

I also tried to add `def` input/output type inference, but unfortunately
we only know the predecl types universally, which means we won't have
enough information to properly know what the types of the custom
commands are.

# User-Facing Changes

Script typechecking will get tighter in some cases
Hovers should be more accurate in some cases that previously resorted to
any.

# Tests + Formatting
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---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

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

@@ -981,7 +981,7 @@ impl TypeScope {
     pub fn get_previous(&self) -> &Type {
         match self.outputs.last().and_then(|v| v.last()) {
             Some(input) => input,
-            None => &Type::Any,
+            None => &Type::Nothing,
         }
     }
 
@@ -1193,7 +1193,7 @@ impl<'a> StateWorkingSet<'a> {
 
     pub fn add_decl(&mut self, decl: Box<dyn Command>) -> DeclId {
         let name = decl.name().as_bytes().to_vec();
-        let input_type = decl.signature().input_type;
+        let input_type = decl.signature().get_input_type();
 
         self.delta.decls.push(decl);
         let decl_id = self.num_decls() - 1;

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

@@ -214,7 +214,23 @@ impl OverlayFrame {
         if let Some(decl) = self.decls.get(&(name, input) as &dyn DeclKey) {
             Some(*decl)
         } else {
-            self.decls.get(&(name, &Type::Any) as &dyn DeclKey).cloned()
+            // then fallback to not using the input type
+            for decl_key in self.decls.keys() {
+                if decl_key.0 == name {
+                    // FIXME: this fallback may give bad type information
+                    // in the case where no matching type is found. But, at
+                    // least we treat it as a found internal command rather
+                    // than an external command, which would cause further issues
+                    return Some(
+                        *self
+                            .decls
+                            .get(decl_key)
+                            .expect("internal error: found decl not actually found"),
+                    );
+                }
+            }
+
+            None
         }
     }
 }