svelte2tsx warning is confusing if typescript isn't installed

[Rich-Harris]

Describe the bug

I saw this error when running npm run package on a project:

You need to install svelte2tsx >=0.4.1 if you want to generate type definitions

svelte2tsx was installed; the source of the error was that typescript wasn't. We probably need to button up the error checking in https://github.com/sveltejs/kit/blob/master/packages/kit/src/core/make_package/index.js; emit_dts (which imports svelte2tsx) is called before transpile_ts (which imports typescript). This function...

kit/packages/kit/src/core/make_package/index.js

Lines 228 to 240 in a12843c

	async function try_load_svelte2tsx() {
	try {
	const svelte2tsx = (await import('svelte2tsx')).emitDts;
	if (!svelte2tsx) {
	throw new Error('Old svelte2tsx version');
	}
	return svelte2tsx;
	} catch (e) {
	throw new Error(
	'You need to install svelte2tsx >=0.4.1 if you want to generate type definitions'
	);
	}
	}

...could fail for any of the following reasons:

1. typescript (or some other svelte2tsx peer dependency, theoretically) isn't installed
2. svelte2tsx isn't installed
3. svelte2tsx is out of date
4. 🤷

At the moment, all errors are assumed to be 3. Also, it's maybe a bit odd that we throw an error, catch it, then immediately throw a different error, discarding the first.

Reproduction

npm init svelte@next my-package # select 'no' for typescript
cd my-package
npm i
npm run package

Logs

No response

System Info

System:
    OS: macOS 10.15.7
    CPU: (16) x64 Intel(R) Core(TM) i9-9980HK CPU @ 2.40GHz
    Memory: 78.52 MB / 64.00 GB
    Shell: 5.7.1 - /bin/zsh
  Binaries:
    Node: 12.21.0 - ~/.nvm/versions/node/v12.21.0/bin/node
    Yarn: 1.22.10 - ~/.nvm/versions/node/v12.21.0/bin/yarn
    npm: 6.14.11 - ~/.nvm/versions/node/v12.21.0/bin/npm
  Browsers:
    Chrome: 91.0.4472.114
    Firefox: 89.0.2
    Safari: 13.1.3
  npmPackages:
    @sveltejs/kit: next => 1.0.0-next.125
    svelte: ^3.34.0 => 3.38.3

Severity

annoyance

Additional Information

No response