add Number.prototype.toFixed(n)

[drsm]

https://www.ecma-international.org/ecma-262/6.0/#sec-number.prototype.tofixed

[xeioex]

@drsm

Take a look: https://gist.github.com/987c6eb18c7b9f45b305791e018f8116
Welcome to test.

[drsm]

@xeioex

Looks good!
Except the following:

>> (10**21).toFixed()
'1000000000000000000000' // bug 20.1.3.3 8. If x ≥ 10**21, then
>> (10**23).toFixed()
'9.999999999999999e+22' // not sure

vs

root@node:~# eshost -x "print((10**21).toFixed(2));"
#### ch
1e+21

#### jsc
1e+21

#### sm
1e+21

#### v8
1e+21

#### xs
1e+21

root@node:~# eshost -x "print((10**23).toFixed(2));"
#### ch
1e+23

#### jsc
1e+23

#### sm
1e+23

#### v8
1e+23

#### xs
9.999999999999999e+22

UPD:
1.

-    if (njs_slow_path(isnan(number) || fabs(number) > 1e21)) {
+    if (njs_slow_path(isnan(number) || fabs(number) >= 1e21)) {

2. is related to Number.prototype.toString(), so OK.