Optionally preserve trailing spaces in `layoutDiff`

[toinehartman]

Is your feature request related to a problem? Please describe.
Since #2031, we have layoutDiff, a useful tool to compute layout differences between trees. An obvious use case is writing a formatter (#2346, usethesource/rascal-language-servers#677).

Formatting can accept options, one of which is 'preserve/trim trailing spaces' - especially useful when formatting existing code without generating huge diffs. Whether using Rascal's declarative formatting tools (toBox o format) or custom/external ones, layoutDiff is the logical place to do this, in the spirit of its existing copyComments toggle.

Describe the solution you'd like
Add an option preserveTrailingWhitespace = false to layoutDiff, which copies trailing whitespace from the original tree to the modified one on demand.

Describe alternatives you've considered

• Leaving the implementation to DSL developers
• Adding the functionality to Tree2Box

Additional context
Came up during the design of formatting support for DSLs: usethesource/rascal-language-servers#677