Update esbuild config in getting started guide (#450)

Description of changes

When following the getting started guide, I got stuck because esbuild seems to prefer the UMD exports of some transpiled modules. Those can not be statically analyzed and cause runtime errors for missing dependencies - in my case `vscode-json-languageservice` and some of its dependencies.

By telling esbuild to prefer the module field over main, this issue gets avoided.

docs/getting-started.md

@@ -61,6 +61,7 @@ const baseConfig = {
 const extensionConfig = {
   ...baseConfig,
   platform: "node",
+  mainFields: ["module", "main"],
   format: "cjs",
   entryPoints: ["./src/extension.ts"],
   outfile: "./out/extension.js",