Properly support trailingSlash: never with a base

[matthewp]

Changes

• When using trailingSlash: 'never' and a base such as /docs it should allow requests to /docs but not to /docs/.
• This was a bit tricky to fix given how we remove the base in the dev server and then later put it back. That's why this touches a few different files.
• It might be worth in the future creating a class / module or something that sort of encapsulates all of the base, trailingSlash, etc. behavior. And then have everything use that.

Testing

• Unit tests added for the dev server to ensure this works as expected.
• Unit test added for routing.

Docs

N/A, bug fix

[changeset-bot]

🦋 Changeset detected

Latest commit: 30d8992

The changes in this PR will be included in the next version bump.

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[bluwy]

Some small nits but looks good. Vite 4 will also be making some base changes soon which may be relevant here: vitejs/vite#10723