Add task build_dist

[MartinKavik]

Example:

[tasks.build_dist]
description = "Build with wasm-pack in release mode and move important files to dist folder"
dependencies = ["build_release"]
script = [
'''
#!@duckscript
rm -r dist
mkdir dist
cp public dist
cp netlify.toml dist/netlify.toml
cp index.html dist/index.html
cp pkg/package.js dist/pkg/package.js
cp pkg/package_bg.wasm dist/pkg/package_bg.wasm
'''
]