LibJS: Implement parseInt()

Here's a reasonably faithful implementation of ECMAScript 2021 18.2.5.
Some corner cases are not covered, I've left them as FIXME's in the
included unit test.

Also I had to tweak JS::Value::to_i32() to always convert infinity to
zero, which is in accordance with ToInt32 AFAICT.

Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -168,6 +168,7 @@ namespace JS {
     P(padStart)                              \
     P(parse)                                 \
     P(parseFloat)                            \
+    P(parseInt)                              \
     P(pop)                                   \
     P(pow)                                   \
     P(preventExtensions)                     \