PSAvoidTrailingWhitespace: Rule not applied when using formatter + single character lines with trailing whitespace are truncated

PR Summary

Added PSAvoidTrailingWhitespace to the list of rules considered by the formatter (an entry for the rule is still required in the Rules part of settings).

Fixes #1992
Fixed an issue where lines starting with and containing a single character, that have trailing white-space, are truncated when formatted/fixed. See comment in #1992 for explanation.

Fixes #1757

[x] PR has a meaningful title
- Use the present tense and imperative mood when describing your changes
[x] Summarized changes
[x] Change is not breaking
[x] Make sure all .cs, .ps1 and .psm1 files have the correct copyright header
[x] Make sure you've added a new test if existing tests do not effectively test the code changed and/or updated documentation
[x] This PR is ready to merge and is not Work in Progress.
- If the PR is work in progress, please add the prefix WIP: to the beginning of the title and remove the prefix when the PR is ready.