Trim whitespace to right of cursor in some cases

[kbrose]

Checklist

• Relevant issues have been referenced
• CHANGELOG.md has been updated (bullet points added to the Unreleased section)
• Tests have been added, or are not relevant

Description

Whitespace to the right of the cursor will now be deleted when pressing Enter. This only happens if there is non-whitespace located somewhere both to the left and to the right of the cursor.

Examples, as usual, | represents the cursor.

def f(x,| y):        # The extra space IS deleted
  | print(x)         # The space to the right IS NOT deleted - no non-whitespace to the left
    print(y)|<space> # The space to the right IS NOT deleted - no non-whitespace to the right

In this way, both of the following situations end up with the right indentation:

def my_func(arg1,| arg2):


def my_func(arg1, |arg2):

# Both end up here:

def my_func(arg1,
            arg2):

# There may be an extra space after arg1 in the second case. If this is a problem
# for you, then use the files.trimTrailingWhitespace setting in VSCode.

Closes #62