Support multiple files in ADD/COPY instructions

Fixes https://github.com/camdencheek/tree-sitter-dockerfile/issues/12

corpus/add.txt

@@ -25,3 +25,16 @@ ADD --chown=a:b /src ./dst
 		(path)
 		(path)))
 
+==================
+Multiple files
+==================
+
+ADD src1 src2 dst
+
+---
+
+(source_file
+  (add_instruction
+    (path)
+    (path)
+    (path)))

corpus/copy.txt

@@ -25,3 +25,16 @@ COPY --chown=a:b /src ./dst
 		(path)
 		(path)))
 
+==================
+Multiple files
+==================
+
+COPY src1 src2 dst
+
+---
+
+(source_file
+  (copy_instruction
+    (path)
+    (path)
+    (path)))

grammar.js

@@ -68,8 +68,9 @@ module.exports = grammar({
       seq(
         alias(/[aA][dD][dD]/, "ADD"),
         optional($.param),
-        $.path,
-        $._non_newline_whitespace,
+        repeat1(
+          seq($.path, $._non_newline_whitespace)
+        ),
         $.path
       ),
 
@@ -77,8 +78,9 @@ module.exports = grammar({
       seq(
         alias(/[cC][oO][pP][yY]/, "COPY"),
         optional($.param),
-        $.path,
-        $._non_newline_whitespace,
+        repeat1(
+          seq($.path, $._non_newline_whitespace)
+        ),
         $.path
       ),