Fix extract_variable on LetExpr

Example
---
```rust
fn main() {
    if $0let$0 Some(x) = Some(2+2) {}
}
```

**Before this PR**:

```rust
fn main() {
    let $0var_name = let Some(x) = Some(2+2);
    if var_name {}
}
```

**After this PR**:

```rust
fn main() {
    let $0var_name = Some(2+2);
    if let Some(x) = var_name {}
}
```

Author: A4-Tacks

crates/ide-assists/src/handlers/extract_variable.rs

@@ -285,7 +285,7 @@ fn peel_parens(mut expr: ast::Expr) -> ast::Expr {
 /// In general that's true for any expression, but in some cases that would produce invalid code.
 fn valid_target_expr(node: SyntaxNode) -> Option<ast::Expr> {
     match node.kind() {
-        SyntaxKind::PATH_EXPR | SyntaxKind::LOOP_EXPR => None,
+        SyntaxKind::PATH_EXPR | SyntaxKind::LOOP_EXPR | SyntaxKind::LET_EXPR => None,
         SyntaxKind::BREAK_EXPR => ast::BreakExpr::cast(node).and_then(|e| e.expr()),
         SyntaxKind::RETURN_EXPR => ast::ReturnExpr::cast(node).and_then(|e| e.expr()),
         SyntaxKind::BLOCK_EXPR => {
@@ -1403,6 +1403,25 @@ fn main() {
         );
     }
 
+    #[test]
+    fn extract_var_let_expr() {
+        check_assist_by_label(
+            extract_variable,
+            r#"
+fn main() {
+    if $0let$0 Some(x) = Some(2+2) {}
+}
+"#,
+            r#"
+fn main() {
+    let $0var_name = Some(2+2);
+    if let Some(x) = var_name {}
+}
+"#,
+            "Extract into variable",
+        );
+    }
+
     #[test]
     fn extract_var_for_cast() {
         check_assist_by_label(
@@ -1738,6 +1757,14 @@ fn main() {
         check_assist_not_applicable(extract_variable, "fn main() { loop { $0break$0; }; }");
     }
 
+    #[test]
+    fn extract_var_for_let_expr_not_applicable() {
+        check_assist_not_applicable(
+            extract_variable,
+            "fn main() { if $0let Some(x) = Some(2+2) {} }",
+        );
+    }
+
     #[test]
     fn extract_var_unit_expr_not_applicable() {
         check_assist_not_applicable(