Url Hinting bug fixes

This is for #6239 to fix hinting for '../' which was exposed by recent file system changes. Added a unit test for this.

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This also add site-root relative path support (i.e. path that start with '/'). Added a unit test for this.

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I also fixed a bug I noticed where list was not filtered for initial list. Recipe:

1. Shutdown and restart Brackets to clear Url Hinting cache
2. Open a .css file in a folder with other files and folders
3. Add this declaration in a rule: `background-image: url("");`
4. Put cursor between quotes and type a single letter

Results:
Hints list shows all files and folders in the current fodler

Expected:
Only files and folders that start with letter typed are listed.

Workaround:
Hit Esc key and then Ctrl-space. This works because it uses cached list, which is filtered.