[RFC] Add support for rollup and taskr

#showerthoughts...

Rollup and Taskr (formerly Fly) have come a long way since Backpack launched. It would be great to support both.

### Why?

There is a lot to be said for zero config. A LOT. Just getting taskr and rollup setup, even for experienced devs, can take 15-30 minutes.

`backpack.config.js` might look like:

```js
// backpack.config.js for rollup

const alias = require('rollup-plugin-alias')
const path = require('path')

module.exports = {
modifyRollupConfig: (config, options, rollup) => {
config.plugins.push(alias({
common: path.join(__dirname, './src/common') . // can now import common from 'common'
}))

return config
}
}
```

```js
// backpack.config.js for taskr
module.exports = {
modifyTaskrConfig: (config, options) => {
config.lint = async function (task) {
await task.source('src/*.js') // etc...
}
return config
}
}
```

Use the proposed backpack.config.js examples as the starting point, then inspect how Backpack currently initializes its build system and where integrations would belong. Define expected behavior for both Rollup and Taskr before implementing support, and add coverage for each configuration path.

Written by the indexing model from the issue text.