Make handling of bigint/numberic unions, overloads more strict

If a string like "100000000000000000000000001" or "3.2" is passed to
a method which has both a numeric and a bigint overload. API users
may expect all cases to magically be handled correctly without
information loss in both cases, but this behavior would require the
type of the result to be based on the contents of the string.

This patch avoids the issue with the following tweaks to BigInt
WebIDL integration:
- bigint and numeric types are marked as "indistinguishable", so it
  is not permitted to have an overload between bigint and a numeric
  type as the distinguishing argument.
- In the es-to-union algorithm, if the type is a union containing
  bigint and a numeric type, and no more specific member applies
  for the conversion directly, then a TypeError is thrown.

Author: littledan

index.bs

@@ -3522,7 +3522,7 @@ the following algorithm returns <i>true</i>.
             <th>numeric types</th>
             <td class="belowdiagonal"></td>
             <td></td>
-            <td>●</td>
+            <td></td>
             <td>●</td>
             <td>●</td>
             <td>●</td>
@@ -8077,9 +8077,11 @@ that correspond to the union’s [=member types=].
         then return the result of
         [=converted to an IDL value|converting=]
         |V| to that type.
-    1.  If |types| includes a [=numeric type=],
-        then return the result of [=converted to an IDL value|converting=]
-        |V| to that [=numeric type=].
+    1.  If |types| includes a [=numeric type=], then:
+        1.  If |types| includes {{bigint}},
+            [=ECMAScript/throw=] a {{ECMAScript/TypeError}}.
+        1.  Return the result of [=converted to an IDL value|converting=]
+            |V| to that [=numeric type=].
     1.  If |types| includes {{boolean}},
         then return the result of [=converted to an IDL value|converting=]
         |V| to {{boolean}}.