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)
```

grammar.js

@@ -7,12 +7,12 @@ function sepBy(sep, rule) {
 }
 
 const PREC = {
-  assign: 6,
-  multiplicative: 5,
-  additive: 4,
-  comparative: 3,
-  and: 2,
-  or: 1,
+  multiplicative: 6,
+  additive: 5,
+  comparative: 4,
+  and: 3,
+  or: 2,
+  assign: 1,
 };
 
 module.exports = grammar({

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": [

test/corpus/regressions.txt

@@ -0,0 +1,16 @@
+================================================================================
+Assignment precedence
+================================================================================
+
+a := b | c
+
+--------------------------------------------------------------------------------
+
+(source_file
+  (assignment_expression
+    (identifier)
+    (operator)
+    (binary_expression
+      (identifier)
+      (operator)
+      (identifier))))