readme: use new light/dark mode feature

Author: evanw

README.md

@@ -1,5 +1,6 @@
 <p align="center">
-  <img src="./images/wordmark.svg" alt="esbuild: An extremely fast JavaScript bundler">
+  <img src="./images/wordmark-light.svg#gh-light-mode-only" alt="esbuild: An extremely fast JavaScript bundler">
+  <img src="./images/wordmark-dark.svg#gh-dark-mode-only" alt="esbuild: An extremely fast JavaScript bundler">
   <br>
   <a href="https://esbuild.github.io/">Website</a> |
   <a href="https://esbuild.github.io/getting-started/">Getting started</a> |
@@ -13,7 +14,8 @@
 Our current build tools for the web are 10-100x slower than they could be:
 
 <p align="center">
-  <img src="images/benchmark.svg" alt="Bar chart with benchmark results">
+  <img src="images/benchmark-light.svg#gh-light-mode-only" alt="Bar chart with benchmark results">
+  <img src="images/benchmark-dark.svg#gh-dark-mode-only" alt="Bar chart with benchmark results">
 </p>
 
 The main goal of the esbuild bundler project is to bring about a new era of build tool performance, and create an easy-to-use modern bundler along the way.