New pref to allow tabbing over whitespace during syntax test development

[keith-hall]

Quite often when writing syntax tests, I don't add any assertions for whitespace (unless I am adding a negation for the previous assertion to confirm the scope is no longer active), as whitespaces typically don't receive any special scopes. Therefore, when I have a file like:

line being tested
# ^^ example1

and I create a new test line and press Tab, I then have to press Space^ to get assertions for the next non-whitespace span.

line being tested
# ^^ example1
#   ^ base_scope

This PR allows me to set a new preference called syntax_test.skip_whitespace to true, so that I can just press Tab and end up with:

line being tested
# ^^ example1
#    ^^^^^ example2

Hopefully other people will also find this useful :)