Introduce character escaping for "extract strings" code action

[MatejKastak]

Related to this feature https://github.com/avast/yls/blob/master/yls/code_actions.py#L38

When extracting strings we don't check/convert the string if it contains characters that needs escaping.

Thinking about this and after discussion with users, we should treat the input strings as un-escaped by default. This means that every " should be changed to \" and also for every \ we should add another one \\.

Some examples (good test cases):

this "is" a test   -> $s00 = "this\"is\" a test"
"this "is" a test" -> $s00 = "this\"is\" a test"
test\              -> $s00 = "test\\"
C:\\Program\       -> $s00 = "C:\\\\Program\\"