Numeric literal leading underscores C# 7.2readability

Allow underscores to be used before the first digit in a numeric literal.

C# 7.0 gave us digit separators ^{C# 7.0} but did not permit them to be used as the first digit in a literal.

C# 7.2 permits the _ character to also be used before the first numeric digit.

Code

NowBefore

C#

const utf16littleEndian = 0x_FE_FF;
const utf16bigEndian    = 0x_FF_FE;

C#

const utf16littleEndian = 0xFE_FF;
const utf16bigEndian    = 0xFF_FE;

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