gmlgo: gmlgo generate should update .gitignore

[silbinarywolf]

The problem
We currently have the following pattern in our .gitignore files for game projects

## GMLGO BEGIN ##
# Ignore compiled asset data
asset/**/*.data
asset/font/data/*.rtf#
# Ignore code generated files
gmlgo_gen.go
**/gmlgo_gen.go
# Ignore Windows build
*.exe
# Ignore GopherJS build
*.js
*.js.map
# Ignore WebAssembly build
*.wasm
# Ignore binary for Linux / Mac
/worm
# Ignore dist files (gmlgo publish)
/dist
## GMLGO END ##

However, we recently added asset/manifest.json to the list. It would be nice if gmlgo generate / gmlgo build updated this file for us, only touching the data inbetween the ## GMLGO START ## tags.