feat(asset): allow non-existent url

[bluwy]

Description

Fixes #4308

Allow non-existent urls in new URL(..., import.meta.url) if it needs to be resolved in runtime.

Additional context

---

What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

Before submitting the PR, please make sure you do the following

• Read the Contributing Guidelines.
• Read the Pull Request Guidelines and follow the Commit Convention.
• Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• Ideally, include relevant tests that fail without this PR but pass with it.

[patak-dev]

The issue I see is that we are no longer going to inform the user of typos when building the app. How usual it is to have URL that aren't known during build time? I think that we should allow them, but maybe we could log a message for them? Something like:

{nonExistentUrl} doesn't exist at build time, it will remain unchanged to be resolved at runtime

[bluwy]

That's sounds like a fair thing to do. If someone wants to turn that off, we could explore vite-ignore in the future too.

[bluwy]

Added a log. It doesn't look that great:

vite v2.9.0-beta.0 building for production...
transforming (1) index.htmlnew URL('non-existent', import.meta.url) doesn't exist at build time, it will remain unchanged to be resolved at runtime
✓ 20 modules transformed.
dist/foo/assets/asset.b9f46fb3.png            12.47 KiB
dist/foo/assets/fragment.d2571ff2.svg         1.11 KiB
dist/foo/assets/Inter-Italic.bab4e808.woff2   105.64 KiB
dist/foo/assets/fragment-bg.c8ba701c.svg      1.13 KiB
dist/foo/assets/Inter-Italic.7b187d57.woff    140.11 KiB
dist/foo/index.html                           20.05 KiB
dist/foo/manifest.json                        0.38 KiB
dist/foo/assets/index.70c90341.js             12.41 KiB / gzip: 4.92 KiB
dist/foo/assets/index.69eb903c.css            19.58 KiB / gzip: 3.90 KiB

It doesn't go in a new line, and I don't think there's a pattern like this too in the codebase (?). Perhaps this is still acceptable.

[patak-dev]

Uh, merged before seeing your comment, we should later fix the new line