feat: convert to TypeScript

```
pnpm i -g jsdoc2flow @khanacademy/flow-to-ts
jsdoc2flow -i .\lib --overwrite
flow-to-ts .\lib\*.js --prettier --write --delete-source
mv ./lib ./src
```

Author: aminya

.gitignore

@@ -8,6 +8,9 @@ pids
 *.seed
 .tmp
 
+# typescript output
+lib/
+
 # Directory for instrumented libs generated by jscoverage/JSCover
 lib-cov