refactor(completion): expression based variable/cell_path completion (#15033)

# Description

fixes #14643 , as well as some nested cell path cases:

```nushell
let foo = {a: [1 {a: 1}]}

$foo.a.1.#<tab>

const bar = {a: 1, b: 2}
$bar.#<tab>
```

So my plan of the refactoring process is that:
1. gradually move those rules of flattened shapes into expression match
branches, until they are gone
2. keep each PR focused, easier to review and track. 

# User-Facing Changes

# Tests + Formatting

+2

# After Submitting

crates/nu-cli/tests/completions/mod.rs

@@ -1554,6 +1554,58 @@ fn variables_completions() {
     match_suggestions(&expected, &suggestions);
 }
 
+#[test]
+fn record_cell_path_completions() {
+    let (_, _, mut engine, mut stack) = new_engine();
+    let command = r#"let foo = {a: [1 {a: 2}]}; const bar = {a: [1 {a: 2}]}"#;
+    assert!(support::merge_input(command.as_bytes(), &mut engine, &mut stack).is_ok());
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+
+    let expected: Vec<String> = vec!["a".into()];
+    let completion_str = "$foo.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+
+    let completion_str = "$foo.a.1.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+
+    let completion_str = "$bar.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+
+    let completion_str = "$bar.a.1.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+
+    let completion_str = "{a: [1 {a: 2}]}.a.1.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+}
+
+#[test]
+fn table_cell_path_completions() {
+    let (_, _, mut engine, mut stack) = new_engine();
+    let command = r#"let foo = [{a:{b:1}}, {a:{b:2}}]; const bar = [[a b]; [1 2]]"#;
+    assert!(support::merge_input(command.as_bytes(), &mut engine, &mut stack).is_ok());
+    let mut completer = NuCompleter::new(Arc::new(engine), Arc::new(stack));
+
+    let expected: Vec<String> = vec!["a".into()];
+    let completion_str = "$foo.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+
+    let expected: Vec<String> = vec!["b".into()];
+    let completion_str = "$foo.a.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+
+    let expected: Vec<String> = vec!["a".into(), "b".into()];
+    let completion_str = "$bar.";
+    let suggestions = completer.complete(completion_str, completion_str.len());
+    match_suggestions(&expected, &suggestions);
+}
+
 #[test]
 fn alias_of_command_and_flags() {
     let (_, _, mut engine, mut stack) = new_engine();