chore: join URL segments more safely

[benmccann]

Description

We are appending the URL path to base path in numerous locations assuming that the base path ends with /. I would like to stop enforcing that the base path ends with / (see #9236). This change implements much of what would be necessary, but is something that could be reviewed immediately and wouldn't have to wait for 4.0.

I introduced a new helper called joinUrlSegments. In most of the cases, this helper wouldn't be necessary and we could just use path.posix.join. However, there's a couple cases where this helper might be preferred. E.g. if the URL includes the scheme then you cannot use path.posix.join because // would be collapsed to /. Also, if the base URL is ./ it would be replaced with an empty string which may not be safe.

Additional context

I did not expose joinUrlSegments, but we may wish to do that and encourage frameworks to use that method. This could be done in Vite 3.2. Frameworks that use this method will find it easier to migrate to Vite 4 assuming we make a breaking change and stop enforcing that URLs end with /.

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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