Support emitting numerics with separators as-is in target: esnext

[saschanaz]

TypeScript Version: 4.1.0-beta (playground)

Search Terms: numeric separator

Code

// A *self-contained* demonstration of the problem follows...
// Test this by running `tsc` on the command-line, rather than through another build tool such as Gulp, Webpack, etc.
2021_10_26

Expected behavior: 2021_10_26

Actual behavior: 20211026

Playground Link: https://www.typescriptlang.org/play?target=99&ts=4.1.0-beta#code/EwBmEYH1xTgNiA

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