LibJS: Implement Number.prototype.toPrecision

As noted in the prototype comments, this implementation becomes less
accurate as the precision approaches the limit of 100. For example:

    (3).toPrecision(100)

Should result in "3." followed by 99 "0"s. However, due to the loss of
accuracy in the floating point computations, we currently result in
"2.9999999...".

Userland/Libraries/LibJS/Runtime/CommonPropertyNames.h

@@ -468,6 +468,7 @@ namespace JS {
     P(toPlainMonthDay)                       \
     P(toPlainTime)                           \
     P(toPlainYearMonth)                      \
+    P(toPrecision)                           \
     P(toString)                              \
     P(total)                                 \
     P(toTemporalInstant)                     \