Compile only canonical schemd fences

Parse the information string before the body

The information string goes to parseSchematicFence before the body is touched. When it returns undefined, the block belongs to someone else and goes straight back to the host parser. Otherwise the body is compiled and SchematicSyntaxError.line is reported. Swallowing a broken example only means shipping it.

const fence = parseSchematicFence(token.info);
if (!fence) return next(token);
return compileSchematic(token.text, { ...fence, mode: 'default' }).svg;

source:VY "3.3 V" at (80, 170) #blue [type=voltage-dc orientation=up]
resistor:RTH "NTC 10 k\Omega" at (300, 90) #amber [type=thermistor]
junction:NTC at (500, 90) #cyan
resistor:RBT "10 k\Omega" at (500, 230) #amber [orientation=down]
port:ADC "to ADC" at (680, 90) #emerald
ground:GY "0 V" at (500, 400) #slate

VY.positive -> RTH.in #blue [ortho]
RTH.out -> NTC.node #amber [ortho]
NTC.node -> RBT.in #cyan [ortho]
NTC.node -> ADC.in #emerald [ortho marker-end=arrow]
RBT.out -> GY.in #slate [ortho]
VY.negative -> GY.in #slate [ortho]

This site compiles every versioned fence in its own test suite, and that is what keeps a documentation example from being decoration. Each docs line keeps its own examples. Newer syntax is never added to an older line, so the 1.2 pages are the only ones that leave labels out.