Fix assignment priority

The assignment operator is lower priority than any other binary
operator.

Otherwise the following:

```tiger
a := b | c
```

Would be parsed as:

```tiger
(a := b) | c
```

Instead of the expected:

```tiger
a := (b | c)
```

src/grammar.json

@@ -412,7 +412,7 @@
       "members": [
         {
           "type": "PREC_LEFT",
-          "value": 5,
+          "value": 6,
           "content": {
             "type": "SEQ",
             "members": [
@@ -459,7 +459,7 @@
         },
         {
           "type": "PREC_LEFT",
-          "value": 4,
+          "value": 5,
           "content": {
             "type": "SEQ",
             "members": [
@@ -506,7 +506,7 @@
         },
         {
           "type": "PREC_LEFT",
-          "value": 3,
+          "value": 4,
           "content": {
             "type": "SEQ",
             "members": [
@@ -569,7 +569,7 @@
         },
         {
           "type": "PREC_LEFT",
-          "value": 2,
+          "value": 3,
           "content": {
             "type": "SEQ",
             "members": [
@@ -607,7 +607,7 @@
         },
         {
           "type": "PREC_LEFT",
-          "value": 1,
+          "value": 2,
           "content": {
             "type": "SEQ",
             "members": [
@@ -830,7 +830,7 @@
     },
     "assignment_expression": {
       "type": "PREC_RIGHT",
-      "value": 6,
+      "value": 1,
       "content": {
         "type": "SEQ",
         "members": [