react-jsx does not work with Preact

Hi

## Error
I've recently made the switch to microbundle for a project that uses Preact and TypeScript.
Following the docs, and me having the latest version of TypeScript, I put the following in my tsconfig:

```json
{
"compilerOptions": {
"jsx": "react-jsx",
"jsxImportSource": "preact",
//...
}
}
```

However when building with microbundle I get the following error:
`rpt2: options error TS5089: Option 'jsxFactory' cannot be specified when option 'jsx' is 'react-jsx'.`
This seems weird behaviour as I have not specified the jsxFactory.

Combined with this I also get an error about the 'jsxFragmentFactory' having to be provided to use JSX Fragments, but I presume that is due to the fact the the jsxFactory is somehow incorrect.

I tried hardcoding the values from the config in my script (without success) by doing the following

`microbundle --jsx react-jsx --jsxImportSource preact`

## Possible solution

I've been able to get microbundle working if I switch back to the tsconfig for older versions of TypeScript, so I'll be opting into this behaviour for now. For reference this is what the old config is:

```json
{
"compilerOptions": {
"jsx": "react",
"jsxFactory": "h",
"jsxFragmentFactory": "Fragment",
//...
}
}
```