Add support for case fallthrough (#85)

Fixes https://github.com/tree-sitter/tree-sitter-bash/issues/40

corpus/statements.txt

@@ -217,7 +217,11 @@ case "opt" in
     ;;
 
   b)
-    echo b;;
+    echo b
+    ;&
+
+  c)
+    echo c;;
 esac
 
 case "$Z" in
@@ -234,6 +238,8 @@ esac
 
 (program
   (case_statement (string)
+    (case_item (word)
+      (command (command_name (word)) (word)))
     (case_item (word)
       (command (command_name (word)) (word)))
     (case_item (word)

grammar.js

@@ -190,7 +190,10 @@ module.exports = grammar({
       repeat(seq('|', field('value', $._literal))),
       ')',
       optional($._statements),
-      prec(1, ';;')
+      prec(1, choice(
+        field('termination', ';;'),
+        field('fallthrough', choice(';&', ';;&'))
+      ))
     ),
 
     last_case_item: $ => seq(

src/grammar.json

@@ -697,8 +697,34 @@
           "type": "PREC",
           "value": 1,
           "content": {
-            "type": "STRING",
-            "value": ";;"
+            "type": "CHOICE",
+            "members": [
+              {
+                "type": "FIELD",
+                "name": "termination",
+                "content": {
+                  "type": "STRING",
+                  "value": ";;"
+                }
+              },
+              {
+                "type": "FIELD",
+                "name": "fallthrough",
+                "content": {
+                  "type": "CHOICE",
+                  "members": [
+                    {
+                      "type": "STRING",
+                      "value": ";&"
+                    },
+                    {
+                      "type": "STRING",
+                      "value": ";;&"
+                    }
+                  ]
+                }
+              }
+            ]
           }
         }
       ]

src/node-types.json

@@ -322,6 +322,30 @@
     "type": "case_item",
     "named": true,
     "fields": {
+      "fallthrough": {
+        "multiple": false,
+        "required": false,
+        "types": [
+          {
+            "type": ";&",
+            "named": false
+          },
+          {
+            "type": ";;&",
+            "named": false
+          }
+        ]
+      },
+      "termination": {
+        "multiple": false,
+        "required": false,
+        "types": [
+          {
+            "type": ";;",
+            "named": false
+          }
+        ]
+      },
       "value": {
         "multiple": true,
         "required": true,
@@ -1403,10 +1427,18 @@
     "type": ";",
     "named": false
   },
+  {
+    "type": ";&",
+    "named": false
+  },
   {
     "type": ";;",
     "named": false
   },
+  {
+    "type": ";;&",
+    "named": false
+  },
   {
     "type": "<",
     "named": false