“syntaxes: support [[SOME_COMPOSITE_NODE]], deprecate SECTION”

[mettta]

• adds support for composite nodes with double brackets
• makes reserved SECTION deprecated
• removes deprecated FREETEXT
• adds support for NODE_IN_TOC; makes REQUIREMENT_IN_TOC deprecated
• adds support for VIEW_STYLE; makes REQUIREMENT_STYLE deprecated
• adds support for PREFIX; makes REQ_PREFIX deprecated
• prevent partial 'METADATA' from ending OPTIONS block
• updates UID match pattern to match symbols like (), /, :, and space
• fix TAG regex

Closes #20
Closes #22
Closes #23