handle relative URLs for resolveURL

[pi0]

Depends on #35, #36.

previous work: #15

resolveURL('foo/bar', '../baz') = 'foo/baz'

[shtse8]

why not baz? because the first string without trailing slash. it may be a filename but not directory

foo/bar + ../ =

+ baz = baz

[pi0]

in URL spec, file and directory are both part of path. This behavior is also consistent with URL:

// file://some/path/bar
new URL('../bar', 'file://some/path/foo/bar').toString()