Change to target: es5 instead of es6

Using es5 is 6x faster!

Here es6 runs at 75,000 eps:
```
$ yarn runit 1000000
yarn runit v0.24.6
$ yarn build && yarn doit 1000000
yarn build v0.24.6
$ tsc -p src/tsconfig.json
Done in 1.97s.
yarn doit v0.24.6
$ node build/test-nn.js 1000000
Epoch=1,000,000 Error=4.63e-7 time=13.17s eps=75,902

Pat	Input0	Input1	Target0	Output0
0	0	0	0	0.00044897833262192035
1	1	0	1	0.9995443735793996
2	0	1	1	0.9995443229790881
3	1	1	0	0.0005559018544514654
Done in 13.34s.
Done in 15.69s.
```
And es5 is 480,000 eps:
```
$ yarn runit 1000000
yarn runit v0.24.6
$ yarn build && yarn doit 1000000
yarn build v0.24.6
$ tsc -p src/tsconfig.json
Done in 1.98s.
yarn doit v0.24.6
$ node build/test-nn.js 1000000
Epoch=1,000,000 Error=4.63e-7 time=2.07s eps=482,894

Pat	Input0	Input1	Target0	Output0
0	0	0	0	0.00044897833262192035
1	1	0	1	0.9995443735793996
2	0	1	1	0.9995443229790881
3	1	1	0	0.0005559018544514654
Done in 2.24s.
Done in 4.60s.
```

tsconfig.base.json

@@ -1,6 +1,6 @@
 {
   "compilerOptions": {
-    "target": "es6",
+    "target": "es5",
     "module": "commonjs",
     "moduleResolution": "Node",
     "sourceMap": true