cmd/dev: run the server & watch for changes

[arschles]

to replace the makefile. it should

• build the wasm app
• build the server
• run the server
• watch changed files (that's prob for a follow-up)

[ghost]

is there any interest in using mage, instead of a makefile. https://github.com/magefile/mage

Mostly because it allow make file functionaity on Windows but also its easier to manage large projects with.

Its used by tons of projects: https://github.com/search?o=desc&q=magefile%2Fmage&s=indexed&type=Code