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-parser/tests/test_parser.rs

@@ -1361,8 +1361,7 @@ mod input_types {
 
         fn signature(&self) -> nu_protocol::Signature {
             Signature::build(self.name())
-                .input_type(Type::Any)
-                .output_type(Type::Custom("custom".into()))
+                .input_output_type(Type::Any, Type::Custom("custom".into()))
                 .category(Category::Custom("custom".into()))
         }
 
@@ -1393,8 +1392,7 @@ mod input_types {
             Signature::build(self.name())
                 .required("column", SyntaxShape::String, "column name")
                 .required("other", SyntaxShape::String, "other value")
-                .input_type(Type::Custom("custom".into()))
-                .output_type(Type::Custom("custom".into()))
+                .input_output_type(Type::Custom("custom".into()), Type::Custom("custom".into()))
                 .category(Category::Custom("custom".into()))
         }
 
@@ -1424,7 +1422,7 @@ mod input_types {
         fn signature(&self) -> nu_protocol::Signature {
             Signature::build(self.name())
                 .required("operation", SyntaxShape::String, "operation")
-                .input_type(Type::Custom("custom".into()))
+                .input_output_type(Type::Custom("custom".into()), Type::Custom("custom".into()))
                 .category(Category::Custom("custom".into()))
         }
 
@@ -1481,8 +1479,7 @@ mod input_types {
         fn signature(&self) -> nu_protocol::Signature {
             Signature::build(self.name())
                 .rest("operation", SyntaxShape::Any, "operation")
-                .input_type(Type::Custom("custom".into()))
-                .output_type(Type::Custom("custom".into()))
+                .input_output_type(Type::Custom("custom".into()), Type::Custom("custom".into()))
                 .category(Category::Custom("custom".into()))
         }
 
@@ -1511,8 +1508,7 @@ mod input_types {
 
         fn signature(&self) -> nu_protocol::Signature {
             Signature::build(self.name())
-                .input_type(Type::Custom("custom".into()))
-                .output_type(Type::Custom("custom".into()))
+                .input_output_type(Type::Custom("custom".into()), Type::Custom("custom".into()))
                 .category(Category::Custom("custom".into()))
         }
 
@@ -1619,7 +1615,7 @@ mod input_types {
                 },
             ) => {
                 let expected_id = working_set
-                    .find_decl(b"ls", &Type::Any)
+                    .find_decl(b"ls", &Type::Nothing)
                     .expect("Error merging delta");
                 assert_eq!(call.decl_id, expected_id)
             }
@@ -1789,7 +1785,7 @@ mod input_types {
                     ..
                 },
             ) => {
-                let expected_id = working_set.find_decl(b"ls", &Type::Any).unwrap();
+                let expected_id = working_set.find_decl(b"ls", &Type::Nothing).unwrap();
                 assert_eq!(call.decl_id, expected_id)
             }
             _ => panic!("Expected expression Call not found"),