[lit-starter-ts] Missing prismjs theme

### Which package(s) are affected?

Starter Kits (@lit/lit-starter-js / @lit/lit-starter-ts)

### Description

When I want to build the docs through `pnpm run docs` the command fails due to missing `prismjs` theme.

```bash
> @lit/lit-starter-ts@2.0.1 docs:assets
> cp node_modules/prismjs/themes/prism-okaidia.css docs/

cp: node_modules/prismjs/themes/prism-okaidia.css: No such file or directory
```

### Reproduction

```bash
git clone https://github.com/lit/lit-element-starter-ts
cd lit-element-starter-ts
pnpm i
pnpm run docs
```

### Workaround

Installing `prismjs` as dev dependency.

### Is this a regression?

No or unsure. This never worked, or I haven't tried before.

### Affected versions

lit 3.1.2

### Browser/OS/Node environment

node v21.6.2
pnpm v8.15.1

Start in the Starter Kit package scripts used by `pnpm run docs`, especially the `docs:assets` command that copies `prismjs/themes/prism-okaidia.css`, and inspect the dependency declarations. Verify the fix by following the reported clone and install steps; done means `pnpm run docs` completes without the missing-theme error.

Written by the indexing model from the issue text.