Missing dependency on preact / react (ghost dependency)

This module tries to import / require from `preact`, but does not declare it as a dependency in its `package.json` (a [ghost dependency](https://yarnpkg.com/features/pnp#ghost-dependencies-protection)). This is an error, [yarn requires all dependency to be listed properly](https://yarnpkg.com/configuration/yarnrc#packageExtensions):

```
Module not found: Error: Can't resolve 'preact' in 'C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact'
resolve 'preact' in 'C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact'
Parsed request is a module
using description file: C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact\package.json (relative path: .)
Field 'browser' doesn't contain a valid alias configuration
resolve as module
request is not managed by the pnpapi
htm tried to access preact, but it isn't declared in its dependencies; this makes the require call ambiguous and unsound.
Required package: preact
Required by: htm@npm:3.1.1 (via C:\Users\user\AppData\Local\Yarn\Berry\cache\htm-npm-3.1.1-e3b831f850-10.zip\node_modules\htm\preact\)
@ ../../../../../../AppData/Local/Yarn/Berry/cache/@bpmn-io-diagram-js-ui-npm-0.2.3-01795dd8e0-10.zip/node_modules/@bpmn-io/diagram-js-ui/lib/index.js 1:0-34 1:0-34
@ ../../../../../../AppData/Local/Yarn/Berry/cache/diagram-js-npm-15.2.4-de7db86f90-10.zip/node_modules/diagram-js/lib/ui/index.js 1:0-39 1:0-39
@ ../../../../../../AppData/Local/Yarn/Berry/cache/diagram-js-npm-15.2.4-de7db86f90-10.zip/node_modules/diagram-js/lib/features/popup-menu/PopupMenu.js 1:0-4:18 132:2-8 133:4-8 278:2-8
@ ../../../../../../AppData/Local/Yarn/Berry/cache/diagram-js-npm-15.2.4-de7db86f90-10.zip/node_modules/diagram-js/lib/features/popup-menu/index.js 1:0-36 12:23-32
@ ../../../../../../AppData/Local/Yarn/Berry/cache/bpmn-js-npm-18.2.0-5474b88486-10.zip/node_modules/bpmn-js/lib/features/align-elements/index.js 3:0-65 13:4-19
@ ../../../../../../AppData/Local/Yarn/Berry/cache/bpmn-js-npm-18.2.0-5474b88486-10.zip/node_modules/bpmn-js/lib/Modeler.js 12:0-60 167:2-21
@ ./src/bpmn-editor/render-bpmn-editor.ts 7:0-48 33:31-41
@ ./src/bpmn-editor/index.ts 1:0-40 1:0-40
```

The same goes for `react`. Possible ways how the dependencies could be fixed:

* declare both react and preact as dependencies (will always install both preact and react)
* declare both react and preact as optional peer dependencies (preferred) ~~(will force users to install both react and preact)~~
* ~~create 2 separate NPM packages, so that each package can declare its dependencies properly~~

# Workaround

For people affected: You can add the following [bandage aid](https://yarnpkg.com/configuration/yarnrc#packageExtensions) to fix the declared dependencies (file `.yarnrc.yml`):

```yaml
packageExtensions:
htm@*:
peerDependencies:
preact: "*"
react: "*"
peerDependenciesMeta:
preact:
optional: true
react:
optional: true
```