Introduce character escaping for "extract strings" code action

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\\"
```

Start in yls/code_actions.py around line 38, where the extract-strings code action is implemented. Use the issue's quoted-string and backslash examples as test cases, and consider the work complete when extracted strings escape every double quote and backslash as shown.

Written by the indexing model from the issue text.