extras: Allow superscript to begin with plus, minus, or single quote

This is admittedly a trick, but it is (a) isolated to superscript
parsing, and (b) only applicable to the plus, minus, and single quote
symbols.

The goldmark parser.ScanDelimiter function receives the line, but
doesn't pass it on to anything else. So all we're doing is replacing the
plus, minus, or single quote symbol with the letter "z" if the symbol is
the first character after the delimiter. Using the letter "z" was an
arbitrary choice.

Closes #30

Author: jmooring

extras/_test/superscript.txt

@@ -60,3 +60,24 @@ text[^1] text[^2]
 //- - - - - - - - -//
 <p>text[^1] text[^2]</p>
 //= = = = = = = = = = = = = = = = = = = = = = = =//
+
+10: Superscript is one of +, -, '
+//- - - - - - - - -//
+x^+^ x^-^ x^'^
+//- - - - - - - - -//
+<p>x<sup>+</sup> x<sup>-</sup> x<sup>'</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+11: Superscript begins with one of +, -, '
+//- - - - - - - - -//
+x^+2^ x^-2^ x^'2^
+//- - - - - - - - -//
+<p>x<sup>+2</sup> x<sup>-2</sup> x<sup>'2</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//
+
+12: Superscript ends with one of +, -, '
+//- - - - - - - - -//
+x^2+^ x^2-^ x^2'^
+//- - - - - - - - -//
+<p>x<sup>2+</sup> x<sup>2-</sup> x<sup>2'</sup></p>
+//= = = = = = = = = = = = = = = = = = = = = = = =//

extras/inline.go

@@ -1,6 +1,8 @@
 package extras
 
 import (
+	"slices"
+
 	"github.com/yuin/goldmark"
 	"github.com/yuin/goldmark/ast"
 	"github.com/yuin/goldmark/parser"
@@ -47,7 +49,17 @@ func (s *inlineTagParser) Trigger() []byte {
 func (s *inlineTagParser) Parse(_ ast.Node, block text.Reader, pc parser.Context) ast.Node {
 	before := block.PrecendingCharacter()
 	line, segment := block.PeekLine()
-	node := parser.ScanDelimiter(line, before, s.Number, newInlineTagDelimiterProcessor(s.inlineTag))
+
+	// Issue 30
+	modifiedLine := slices.Clone(line)
+	if s.inlineTag.TagKind == kindSuperscript && len(line) > s.Number {
+		symbols := []byte{'+', '-', '\''}
+		if slices.Contains(symbols, line[s.Number]) {
+			modifiedLine[s.Number] = 'z' // replace with any letter or number
+		}
+	}
+
+	node := parser.ScanDelimiter(modifiedLine, before, s.Number, newInlineTagDelimiterProcessor(s.inlineTag))
 	if node == nil || node.OriginalLength > 2 || before == rune(s.Char) {
 		return nil
 	}