Add an error highlighting extension to better visualize errors in coq-lsp.

Dockerfile

 FROM inf110-workspace-base:latest
 
-# Install the coq-lsp extension for code-server
+# Install the coq-lsp extension for code-server and an extension that highlights errors in the code
 RUN code-server --install-extension ejgallego.coq-lsp
+RUN code-server --install-extension usernamehw.errorlens
 
 # Install the Jupyter / Python extensions for code-server
 RUN code-server --install-extension ms-toolsai.jupyter