support multiple parameters to ADD and COPY

This commit adds support for parsing multiple parameters to both `ADD`
and `COPY` commands.

For example, tree-sitter-dockerfile now correctly parses both of these
lines from the dockerfile reference[^1]:

```dockerfile
ADD --chown=myuser:mygroup --chmod=655 files* /somedir/
COPY --chown=myuser:mygroup --chmod=644 files* /somedir/
```

Closes #43

[^1]: https://docs.docker.com/engine/reference/builder/

corpus/add.txt

@@ -38,3 +38,18 @@ ADD src1 src2 dst
     (path)
     (path)
     (path)))
+
+==================
+Multiple params
+==================
+
+ADD --chown=a:b --chmod=644 src dst
+
+---
+
+(source_file
+  (add_instruction
+    (param)
+    (param)
+    (path)
+    (path)))

corpus/copy.txt

@@ -38,3 +38,18 @@ COPY src1 src2 dst
     (path)
     (path)
     (path)))
+
+==================
+Multiple params
+==================
+
+COPY --chown=a:b --chmod=644 src dst
+
+---
+
+(source_file
+  (copy_instruction
+    (param)
+    (param)
+    (path)
+    (path)))

grammar.js

@@ -73,7 +73,7 @@ module.exports = grammar({
     add_instruction: ($) =>
       seq(
         alias(/[aA][dD][dD]/, "ADD"),
-        optional($.param),
+        repeat($.param),
         repeat1(
           seq($.path, $._non_newline_whitespace)
         ),
@@ -83,7 +83,7 @@ module.exports = grammar({
     copy_instruction: ($) =>
       seq(
         alias(/[cC][oO][pP][yY]/, "COPY"),
-        optional($.param),
+        repeat($.param),
         repeat1(
           seq($.path, $._non_newline_whitespace)
         ),

src/grammar.json

@@ -336,16 +336,11 @@
           "value": "ADD"
         },
         {
-          "type": "CHOICE",
+          "type": "REPEAT",
-          "members": [
+          "content": {
-            {
+            "type": "SYMBOL",
-              "type": "SYMBOL",
+            "name": "param"
-              "name": "param"
+          }
-            },
-            {
-              "type": "BLANK"
-            }
-          ]
         },
         {
           "type": "REPEAT1",
@@ -382,16 +377,11 @@
           "value": "COPY"
         },
         {
-          "type": "CHOICE",
+          "type": "REPEAT",
-          "members": [
+          "content": {
-            {
+            "type": "SYMBOL",
-              "type": "SYMBOL",
+            "name": "param"
-              "name": "param"
+          }
-            },
-            {
-              "type": "BLANK"
-            }
-          ]
         },
         {
           "type": "REPEAT1",