Fix bun.String.toOwnedSliceReturningAllASCII (#23925)

`bun.String.toOwnedSliceReturningAllASCII` is supposed to return a
boolean indicating whether or not the string is entirely composed of
ASCII characters. However, the current implementation frequently
produces incorrect results:

* If the string is a `ZigString`, it always returns true, even though
`ZigString`s can be UTF-16 or Latin-1.
* If the string is a `StaticZigString`, it always returns false, even
though `StaticZigStrings` can be all ASCII.
* If the string is a 16-bit `WTFStringImpl`, it always returns false,
even though 16-bit `WTFString`s can be all ASCII.
* If the string is empty, it always returns false, even though empty
strings are valid ASCII strings.

`toOwnedSliceReturningAllASCII` is currently used in two places, both of
which assume its answer is accurate:

* `bun.webcore.Blob.fromJSWithoutDeferGC`
* `bun.api.ServerConfig.fromJS`

(For internal tracking: fixes ENG-21249)

src/string/immutable.zig

@@ -10,6 +10,19 @@ pub const Encoding = enum {
     utf16,
 };
 
+pub const AsciiStatus = enum {
+    unknown,
+    all_ascii,
+    non_ascii,
+
+    pub fn fromBool(is_all_ascii: ?bool) AsciiStatus {
+        return if (is_all_ascii orelse return .unknown)
+            .all_ascii
+        else
+            .non_ascii;
+    }
+};
+
 /// Returned by classification functions that do not discriminate between utf8 and ascii.
 pub const EncodingNonAscii = enum {
     utf8,