feat(backend/sdoc_source_code): Allow empty lines in source node fields

[haxtibal]

Previously, a source node field was terminated at the first empty line. A workaround existed where adding literal \n\n was translated to new line.

This change removes the workaround and introduces a more user friendly rule where empty lines are kept as part of a field. To terminate the field we can either simply start the next field, or insert a reserved termination word that ends the field preliminary.

Comment grammar didn't change much. Most notably

• newlines that belong to field text are now preserved and part of the parse tree
• auto-dedent: the first non-empty line after the field line determines how much indent is automatically removed from each line

Note: The termination symbol is fixed to "SPDX-Req-End", this should be made configurable in an upcoming PR.

[haxtibal]

Please mind the PR description "Most notably newlines that belong to field text are now part of the parse tree".

I.e., line breaks from source node fields are now preserved (it becomes visible when you switch from RST to text rendering). Is it ok? I can adapt our tests, but don't know if it would break users.

I think the change is important in situations like here

 * SPDX-Text:
 * __ftrace_event_enable_disable - enable or disable a trace event.
 * @file: trace event file associated with the event.
 * @enable: 0 or 1 respectively to disable/enable the event (any other value is
 * invalid).
 * @soft_disable: 1 or 0 respectively to mark if the enable parameter IS or
 * IS NOT a soft enable/disable.
 *
 * Function Expectations: [...]

where we have a foreign syntax (kernel-doc) stuffed on top of our node field syntax.

EDIT:
I've also added a trivial field auto-dedent for two reasons:

• the parser preprocessor adds spurious whitespace in the beginning of each line when it replaces comment characters with whitespace; this shall not be part of a field
• users may add anything from 0 to n whitespaces at the beginning by their coding style when they write comments