Preserve base slash when trailingSlash ignore

[bluwy]

Changes

fix #6623

When trailingSlash is set to ignore, the import.meta.env.BASE_URL will have a string that's the same as config.base as-is. Unlike always where it always adds a trailing slash, and unlike never where it'll never have a trailing slash.

Since the issue was created, looks like docs now document the old, different behaviour:

By default, the value of import.meta.env.BASE_URL includes a trailing slash. If you have the trailingSlash option set to 'never', you will need to add it manually in your static asset imports and URLs.

where ignore would work like always (Always have a trailing slash). Perhaps we should discuss if this change makes sense?

Testing

Updated some tests to fix the old behaviour, the rest should still pass.

Docs

Added changelog, and updated docs in astro.ts
/cc @withastro/maintainers-docs for feedback!

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[delucis]

Thanks @bluwy! Left some changeset and docs comments.

[bluwy]

Merging this in as the examples check is blocked by #7998