Function parsePattern

Convert a parsed Backolon pattern block into an internal pattern Thing.

pattern syntax:

• <one space> --> any amount (zero or more) of spaces or newlines
• <two spaces> --> any amount (zero or more) of spaces, but no newlines
• <three spaces> --> one or more spaces (space is required) without newlines
• <newline> --> newline literal
• x --> wildcard capture of any element named x
• x ... --> repeat x (lazy)
• x ... [+] --> repeat x (greedy) where [+] is a square bracket containing +
• (x) --> grouping (parenthesised pattern)
• {x|y} --> alternation (either x or y)
• [x(stuff)] --> capture name with subpattern
• [x: roundblock] --> type match & capture
• [=xyz] --> literal match (can be symbol, number, string)
• [^] and [$] --> anchors
• number or string literal --> not allowed