Allow complex types in input/output and let (#10405)

# Description
This PR fixes #9702 on the side of parse. I.e. input/output types in
signature and type annotations in `let` now should correctly parse with
type annotations that contain commas and spaces:

![image](https://github.com/nushell/nushell/assets/17511668/babc0a69-5cb3-46c2-98ef-6da69ee3d3be)

# User-Facing Changes
Return values and let type annotations now can contain stuff like
`table<a: int b: record<c: string d: datetime>>` e.t.c

tests/parsing/mod.rs

@@ -2,6 +2,7 @@ use nu_test_support::fs::Stub::FileWithContentToBeTrimmed;
 use nu_test_support::playground::Playground;
 use nu_test_support::{nu, pipeline};
 use pretty_assertions::assert_eq;
+use rstest::rstest;
 
 #[test]
 fn source_file_relative_to_file() {
@@ -244,3 +245,29 @@ fn parse_long_duration() {
 
     assert_eq!(actual.out, "1min 18sec 797ms");
 }
+
+#[rstest]
+#[case("def test []: int -> int { 1 }")]
+#[case("def test []: string -> string { 'qwe' }")]
+#[case("def test []: nothing -> nothing { null }")]
+#[case("def test []: list<string> -> list<string> { [] }")]
+#[case("def test []: record<a: int b: int> -> record<c: int e: int> { {c: 1 e: 1} }")]
+#[case("def test []: table<a: int b: int> -> table<c: int e: int> { [ {c: 1 e: 1} ] }")]
+#[case("def test []: nothing -> record<c: int e: int> { {c: 1 e: 1} }")]
+fn parse_function_signature(#[case] phrase: &str) {
+    let actual = nu!(phrase);
+    assert!(actual.err.is_empty());
+}
+
+#[rstest]
+#[case("let a: int = 1")]
+#[case("let a: string = 'qwe'")]
+#[case("let a: nothing = null")]
+#[case("let a: list<string> = []")]
+#[case("let a: record<a: int b: int> = {a: 1 b: 1}")]
+#[case("let a: table<a: int b: int> = [[a b]; [1 2] [3 4]]")]
+#[case("let a: record<a: record<name: string> b: int> = {a: {name: bob} b: 1}")]
+fn parse_let_signature(#[case] phrase: &str) {
+    let actual = nu!(phrase);
+    assert!(actual.err.is_empty());
+}