Introduce negate pattern in files.exclude

[dankox]

This PR fixes #869

Check files.exclude setting and extract all negate patterns from it into separate "include files" list.
To allow ! as a first character of file name to be excluded, user should use backslash:

"files.exclude": {
	"*.js": true,
	"!gulpfile.js": true,
	"\\!my_exclude_file.txt": true  // needs double backslash to backslash the backslash :)
},

[ghost]

All CLA requirements met.

[lramos15]

File excludes are used in more than the explorer such as in search so this change has to be done on the glob parser.

[dankox]

@lramos15 oh I see... and adding similar logic to getExcludes() in search.ts file wouldn't be enough?
I'm just asking, because if it would be, I could look into it too. However, if it's not the way you want it, then feel free to discard this change.

[bpasero]

I suggest to follow through #134415 and also note the branch I had started, maybe pick it up. from there: https://github.com/microsoft/vscode/tree/ben/134415

[dankox]

@bpasero thanks for the heads up. So in such case I'm going to close this PR as I see there is the ongoing implementation of negate-pattern in glob which makes more sense. I might checkout your branch to try it as I was using this one for me internally for now.