Use field instead of variable for instance variable symbol kinds (#2898)

### Motivation

Very minor thing, but I noticed that we were using the wrong symbol kind for instance variables in document symbols. We use `FIELD` everywhere else, so that it doesn't get confused with other types of variables.

### Implementation

Changed to `FIELD`.

### Automated Tests

Updated the tests.

Author: vinistock

lib/ruby_lsp/listeners/document_symbol.rb

@@ -266,7 +266,7 @@ def on_class_variable_write_node_enter(node)
       def on_instance_variable_write_node_enter(node)
         create_document_symbol(
           name: node.name.to_s,
-          kind: Constant::SymbolKind::VARIABLE,
+          kind: Constant::SymbolKind::FIELD,
           range_location: node.name_loc,
           selection_range_location: node.name_loc,
         )
@@ -276,7 +276,7 @@ def on_instance_variable_write_node_enter(node)
       def on_instance_variable_operator_write_node_enter(node)
         create_document_symbol(
           name: node.name.to_s,
-          kind: Constant::SymbolKind::VARIABLE,
+          kind: Constant::SymbolKind::FIELD,
           range_location: node.name_loc,
           selection_range_location: node.name_loc,
         )
@@ -286,7 +286,7 @@ def on_instance_variable_operator_write_node_enter(node)
       def on_instance_variable_or_write_node_enter(node)
         create_document_symbol(
           name: node.name.to_s,
-          kind: Constant::SymbolKind::VARIABLE,
+          kind: Constant::SymbolKind::FIELD,
           range_location: node.name_loc,
           selection_range_location: node.name_loc,
         )
@@ -296,7 +296,7 @@ def on_instance_variable_or_write_node_enter(node)
       def on_instance_variable_and_write_node_enter(node)
         create_document_symbol(
           name: node.name.to_s,
-          kind: Constant::SymbolKind::VARIABLE,
+          kind: Constant::SymbolKind::FIELD,
           range_location: node.name_loc,
           selection_range_location: node.name_loc,
         )

test/expectations/document_symbol/ivar.exp.json

@@ -2,7 +2,7 @@
   "result": [
     {
       "name": "@a",
-      "kind": 13,
+      "kind": 8,
       "range": {
         "start": {
           "line": 0,
@@ -27,7 +27,7 @@
     },
     {
       "name": "@aa",
-      "kind": 13,
+      "kind": 8,
       "range": {
         "start": {
           "line": 1,
@@ -76,7 +76,7 @@
       "children": [
         {
           "name": "@b",
-          "kind": 13,
+          "kind": 8,
           "range": {
             "start": {
               "line": 4,
@@ -101,7 +101,7 @@
         },
         {
           "name": "@bb",
-          "kind": 13,
+          "kind": 8,
           "range": {
             "start": {
               "line": 5,
@@ -150,7 +150,7 @@
           "children": [
             {
               "name": "@c",
-              "kind": 13,
+              "kind": 8,
               "range": {
                 "start": {
                   "line": 8,
@@ -175,7 +175,7 @@
             },
             {
               "name": "@cc",
-              "kind": 13,
+              "kind": 8,
               "range": {
                 "start": {
                   "line": 9,