chore: make UI dev easier

[markphelps]

This takes advantage of https://vitejs.dev/guide/backend-integration.html and allows us to still load the UI on port :8080 when developing while taking advantage of hot reloading provided by Vite.

The instructors are updated in DEVELOPMENT.md, but basically the steps are:

1. Run npm run dev where you have the flipt-ui repository checked out. This will start a development server on port 5173 and proxy API requests to the Flipt API on port 8080.
2. Run mage dev from the this repository. This will create a dev build that will serve the UI from the development server, while still making it accessible on port 8080.
3. Run the binary with the local config: ./bin/flipt --config ./config/local.yml. This will enable CORS with the correct configuration and start the server on port 8080.
4. Visit http://localhost:8080 to see the UI.
5. Any changes made in the flipt-ui repository will be picked up by the development server and the UI will be reloaded.

[codecov-commenter]

Codecov Report

Merging #1278 (b31cf09) into main (5af0757) will decrease coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1278      +/-   ##
==========================================
- Coverage   79.82%   79.63%   -0.19%     
==========================================
  Files          42       42              
  Lines        3217     3217              
==========================================
- Hits         2568     2562       -6     
- Misses        521      525       +4     
- Partials      128      130       +2     

Impacted Files	Coverage Δ
internal/storage/oplock/sql/sql.go	90.82% <0.00%> (-5.51%)	⬇️

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[GeorgeMac]

Neat!