Allow emitFile filenames that start with a period

[BPScott]

The current check for "you can't have absolute or relative paths in the filename property" makes it impossible to create the filename .vite/stats.html even though that not a relative path and thus is a valid filename to pass into emitFile.

This PR updates the check so the relative path checks looks for files that start with ./ or ../ rather than any filename that starts with a .

[btd]

Thank you for PR, Does it work on windows?

[BPScott]

@btd Thanks for the callout. It looks like Rollup doesn't normalize paths. I've updated to an approach using a regex that checks for ./, ../, .\ and ..\.

I've not got a windows machine to easily test this on.