lit-html newline to <br> directive

nl2br.ts

import {directive, NodePart, html} from 'lit-html';

export const nl2br = directive((val: string) => (part: NodePart) => {
  if (!(part instanceof NodePart)) {
    throw new Error('nl2br can only be used in content bindings');
  }

  const lines = val.split('\n');
  const l = lines.length;

  part.setValue(lines.map((line: string, i) => i === l - 1 ? line : html`${line}<br>`));
});