style: add eslint
What
Install typescript-eslint and run in CI.
Why
To not miss any typescript checking and have some consistent code base with linting.
For example we see the following errors right now:
$ npx eslint .
/Users/steve/code/gitlab.com/gitlab-org/cells/router/src/index.ts
21:42 error 'ctx' is defined but never used @typescript-eslint/no-unused-vars
26:7 error 'resolveOverride' is never reassigned. Use 'const' instead prefer-const
✖ 2 problems (2 errors, 0 warnings)
1 error and 0 warnings potentially fixable with the `--fix` option.