[pkg/stanza] Extract trim func from split package

[djaglowski]

Follows #26241

Previously, split funcs were responsible for applying trim funcs. This PR increases composability by applying trim funcs as a wrapper around split funcs.

One nuance that was surfaced here is that the newline split func was not handling the case where a line starts with a newline. When this happens, we need to tell the scanner to advance, but we still want to return a "" token, rather than nil. This is covered by existing tests, but previously it was "fixed" by the trim func which would return an empty slice when the token was nil. Now, the newline split func will explicitly handle this case, while the trim func will return the original value if it is nil or empty.