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-std/std/xml.nu

@@ -19,7 +19,7 @@ export def xaccess [
     # In xpath first element in path is applied to root element
     # this way it is possible to apply first step to root element
     # of nu xml without unrolling one step of loop
-    mut values = ()
+    mut values: any = ()
     $values = {content: [ { content: $input } ] }
     for $step in ($path) {
         match ($step | describe) {
@@ -66,7 +66,7 @@ def xupdate-string-step [ step: string rest: list updater: closure ] {
 
     let new_values = ($to_update.item | xupdate-internal $rest $updater)
 
-    mut reenumerated_new_values = ($to_update.index | zip $new_values | each {|x| {index: $x.0 item: $x.1}})
+    mut reenumerated_new_values: any = ($to_update.index | zip $new_values | each {|x| {index: $x.0 item: $x.1}})
 
     mut new_content = []
     for it in ($input.content | enumerate) {