ES2019 Feature: Numeric Separator

Syntax:

New NumericLiteral syntax that allows for an _ separator in numbers.

let fee = 123_00;   // $123 (12300 cents, apparently)
let fee = 12_300;   // $12,300 (woah, that fee!)
let value = 1_000_000_000;

Grammar changes are to NumericLiteral, allowing the _ separator to show up in numbers.

+NumericLiteralSeparator::
+    _

 DecimalIntegerLiteral::
     0
-    NonZeroDigit DecimalDigits?
+    NonZeroDigit
+    NonZeroDigit NumericLiteralSeparator? DecimalDigits

 DecimalDigits::
     DecimalDigit
-    DecimalDigits DecimalDigit
+    DecimalDigits NumericLiteralSeparator? DecimalDigit

 BinaryDigits::
     BinaryDigit
-    BinaryDigits BinaryDigit
+    BinaryDigits NumericLiteralSeparator? BinaryDigit

 OctalDigits::
     OctalDigit
-    OctalDigits OctalDigit
+    OctalDigits NumericLiteralSeparator? OctalDigit

 HexDigits::
     HexDigit
-    HexDigits HexDigit
+    HexDigits NumericLiteralSeparator? HexDigit

Spec:

TC39:
https://github.com/tc39/proposal-numeric-separator
https://tc39.es/proposal-numeric-separator/

ESTree:
No node changes proposed, numeric Literal node value will just be the value

Additional considerations

• Supported in Chrome, Safari. (Firefox?)
• Supported in Node, jsc
• May have interaction with BigInt (https://github.com/jquery/esprima/issues/1988)

Remaining Tasks:

• Update Scanner (scanner.ts changes for _ numeric separator)
• Provide Unit Tests and Coverage
• Update test262 (~test262.git#2ee3864136) / test262-stream (~1.3.0)

Test Cases

Valid:

1_2_3_4
1_000_000
10000_00
1_000000_0_0_0
1_2.3_4e5_6
1_1.2_2
1_1e2_2
0x1_0
0o1_0
0b1_0

Invalid:

1__1
1.2__2
1e2__2
1_
1_.2
1_e2
1._2
1.2_
1e_2
1e2_
0x_1
0x1__1
0o_1
0o1__1
0b_1
0b1__1
001_2   // Leading Zero (Legacy Hex) does not allow
09_1    // Leading Zero does not allow

Start in scanner.ts, where numeric literal scanning must be updated for the separator rules shown in the issue. Add unit tests covering the listed valid and invalid cases, then update test262 and test262-stream as requested. Done means separators parse only in permitted positions, with the expected errors for invalid forms and coverage for the syntax.