Don't want to write this by hand? GodCraft sets minecraft:light_emission for you with a switch.Get GodCraftOverview
minecraft:light_emission is a block component: it goes in the components object of a custom block. Makes the block glow, from 0 (no light) to 15 (as bright as a torch or glowstone).
Example
Inside the components object of BP/blocks/<name>.json (values are the defaults or placeholders):
BP/blocks/<name>.json
{
"minecraft:light_emission": {}
}