Support old browsers in @vitest/browser

[edoardocavazza]

Clear and concise description of the problem

Right now, the @vitest/browser is built using default vite targets. Since the client library is served through a sirv middleware, it does not get transpiled by the vite server during tests. This lead old browsers without optional chaining support to a syntax error.

Suggested solution

Would you consider to build @vitest/browser using the lowest esbuild target available?

Alternative

Is it possibile to make vite to serve the client library without sirv?

Additional context

I am building a provider for Saucelabs browsers.

Validations

• Follow our Code of Conduct
• Read the Contributing Guidelines.
• Read the docs.
• Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[sheremet-va]

Should be fine as long as they support import syntax.

[sheremet-va]

As a team, we decided it would be a good idea to support this. The browser compatibility table looks like this, so we should follow it instead of arbitrary es2017: https://vitejs.dev/guide/build.html#browser-compatibility

[sheremet-va]

Actually, @vitest/browser assets are now served by the Vite server, not sirv so they follow the Vite target.