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-command/src/database/commands/query_db.rs

@@ -24,8 +24,6 @@ impl Command for QueryDb {
                 SyntaxShape::String,
                 "SQL to execute against the database",
             )
-            .input_type(Type::Any)
-            .output_type(Type::Any)
             .category(Category::Custom("database".into()))
     }
 

crates/nu-command/src/database/commands/schema.rs

@@ -17,8 +17,6 @@ impl Command for SchemaDb {
     fn signature(&self) -> Signature {
         Signature::build(self.name())
             .input_output_types(vec![(Type::Any, Type::Any)])
-            .input_type(Type::Any)
-            .output_type(Type::Any)
             .category(Category::Custom("database".into()))
     }
 

crates/nu-command/src/filters/lines.rs

@@ -23,7 +23,7 @@ impl Command for Lines {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("lines")
-            .input_output_types(vec![(Type::String, Type::List(Box::new(Type::String)))])
+            .input_output_types(vec![(Type::Any, Type::List(Box::new(Type::String)))])
             .switch("skip-empty", "skip empty lines", Some('s'))
             .category(Category::Filters)
     }

crates/nu-command/src/strings/encode_decode/encode_base64.rs

@@ -31,7 +31,6 @@ impl Command for EncodeBase64 {
                 SyntaxShape::CellPath,
                 "For a data structure input, encode data at the given cell paths",
             )
-            .output_type(Type::String)
             .category(Category::Hash)
     }
 

crates/nu-command/src/strings/encode_decode/encode_hex.rs

@@ -22,7 +22,6 @@ impl Command for EncodeHex {
                 SyntaxShape::CellPath,
                 "For a data structure input, encode data at the given cell paths",
             )
-            .output_type(Type::String)
             .category(Category::Formats)
     }