Bundle the extension into one js file with esbuild. (clangd#287)

This reduces vsix size from 1.5MB -> 500KB, and activate time by 60ms or so.

Note that esbuild consumes TS directly rather than running tsc, and
doesn't produce diagnostics. However they still show up in the editor,
and `test` will still fail on tsc compile errors.

Author: sam-mccall

.vscodeignore

@@ -1,8 +1,7 @@
-.*
-.*/**
-out/test/**
-test/**
-src/**
-**/*.map
-tsconfig.json
-DEVELOPING.md
+**
+!*.md
+!*.png
+!doc-assets/**
+!icon.png
+!out/bundle.js
+!package.json