Do not insert a whitespace element as first or last child inside a composite element

[paul-dingemans]

Description

Prevent inserting a whitespace element as first or last element in a composite node. In such cases insert the whitespace just before or after the composite element (recursively if needed).

#2688 shows an example in which two different rules were inserting a whitespace at the same position in the code fragment but at different places in the AST. As a result two spaces were added to the formatted code instead of one.

Closes #2688

Checklist

Before submitting the PR, please check following (checks which are not relevant may be ignored):

• Commit message are well written. In addition to a short title, the commit message also explain why a change is made.
• At least one commit message contains a reference Closes #<xxx> or Fixes #<xxx> (replace<xxx> with issue number)
• Tests are added
• KtLint format has been applied on source code itself and violations are fixed
• PR title is short and clear (it is used as description in the release changelog)
• PR description added (background information)

Documentation is updated. See difference between snapshot and release documentation

• Snapshot documentation in case documentation is to be released together with a code change
• Release documentation in case documentation is related to a released version of ktlint and has to be published as soon as the change is merged to master