We have to remove `--failAfterWarnings` from our Rollup integration-test,
because Typescript, when targeting es5, will use global `this` for transpilation.
Rollup warns about this, since it replaces `this` with `undefined`:

```
src/index.js → dist/bundle.js...
(!) `this` has been rewritten to `undefined`
https://rollupjs.org/guide/en/#error-this-is-undefined
../../../node_modules/@handlebars/parser/dist/esm/printer.js
1: var __spreadArrays = (this && this.__spreadArrays) || function () {}
```

See https://github.com/handlebars-lang/handlebars-parser/releases/tag/v2.0.0

The `browserlist`-config was introduced in 6bc73fd.

See https://webpack.js.org/configuration/target/#browserslist.

Prettier v2 has the following breaking changes:
* enforces spaces between `function` and params
* enforces trailing commas by default

This is needed in order to use `globalThis`, see #1894.
It also made it possible to remove some old polyfills and fallbacks.

Handlebars v5 will target modern browsers,
while older browsers will still receive support
via version 4.x.

* Updated lock-file to fix npm/cli#4859.
* Updated integration-tests to webpack 5 to fix
  webpack/webpack#14532.
* Added `mode` to webpack-integration-tests to avoid
  the warning `The 'mode' option has not been set...`.
* Replaced outdated `grunt-bg-shell`-package to get rid of
  coffee-script warnings

As explained in issue #1844 and in issue
webpack/webpack#15007 (comment),
the way we used the `browser`-field was wrong.

The main reason for using the `browser`-field is the requirement of
`require('fs')` in the main-entry-file.
The workaround for this was using `require('handlebars/lib/handlebars')`,
but now it will also work via `require('handlebars')` for bundlers that
respect the `browser`-field.

The `"./runtime"`-config was removed, because it didn't have any effect.
In order to detect regressions, the webpack-integration test was
extended to test with different webpack versions.

Fixes #1174
Closes #1844

The `eco`-templates (Embedded CoffeeScript templates)
had their last update over 10 years ago, so we can
remove this dependency from our benchmark.

Fixes the following error when running integration-tests:
```
npm ERR! code ERESOLVE
npm ERR! ERESOLVE unable to resolve dependency tree
npm ERR!
npm ERR! While resolving: webpack-test@1.0.0
npm ERR! Found: handlebars@5.0.0-alpha.1
npm ERR! node_modules/handlebars
npm ERR!   dev handlebars@"file:../../.." from the root project
npm ERR!
npm ERR! Could not resolve dependency:
npm ERR! peer handlebars@">= 1.3.0 < 5" from handlebars-loader@1.7.1
npm ERR! node_modules/handlebars-loader
npm ERR!   dev handlebars-loader@"^1.7.1" from the root project
npm ERR!
```

AMD modules were removed in v5.

Closes #1696

Also reorganized npm scripts.

Also removed it from published files, since the script doesn't seem to be used.

Fourth part of reorganizing and cleaning up test-folders.

Third part of reorganizing and cleaning up test-folders.

First part of reorganizing and cleaning up test-folders.