feat: upgrade to linter v2 API

Upgrade to the v2 version of the linter API, allowing this provider to 
continue working with newer versions of linter.

lib/worker.js

@@ -2,7 +2,6 @@
 
 /* global emit */
 
-import escapeHTML from 'escape-html';
 import fs from 'fs';
 import path from 'path';
 import { getRuleUri } from 'tslint-rule-documentation';
@@ -171,6 +170,11 @@ async function getProgram(Linter, configurationPath) {
   return program;
 }
 
+function getSeverity(failure) {
+  const severity = failure.ruleSeverity.toLowerCase();
+  return ['info', 'warning', 'error'].includes(severity) ? severity : 'warning';
+}
+
 /**
  * Lint the provided TypeScript content
  * @param content {string} The content of the TypeScript file
@@ -240,13 +244,16 @@ async function lint(content, filePath, options) {
     const startPosition = failure.getStartPosition().getLineAndCharacter();
     const endPosition = failure.getEndPosition().getLineAndCharacter();
     return {
-      type: failure.ruleSeverity || 'warning',
-      html: `${escapeHTML(failure.getFailure())} (<a href="${ruleUri.uri}">${failure.getRuleName()}</a>)`,
-      filePath: path.normalize(failure.getFileName()),
-      range: [
-        [startPosition.line, startPosition.character],
-        [endPosition.line, endPosition.character],
-      ],
+      severity: getSeverity(failure),
+      excerpt: `${failure.getFailure()} (${failure.getRuleName()})`,
+      url: ruleUri.uri,
+      location: {
+        file: path.normalize(failure.getFileName()),
+        position: [
+          [startPosition.line, startPosition.character],
+          [endPosition.line, endPosition.character],
+        ],
+      },
     };
   });
 }