extras: Export the InlineTag data type

This is a prerequisite for addressing gohugoio/hugo/issues/12605.

extras: Export variables and a method

This is a prerequisite for addressing gohugoio/hugo/issues/12605.

passthrough: Upgrade goldmark and fix upstream regression

* passthrough: Upgrade goldmark and fix upstream regression

Goldmark v1.7.5 changed the default behaviour of the BaseBlock.Text() function, breaking the block transformer for block math. Later, in Goldmark v1.7.8, the whole Text() function was deprecated. This commit also deprecates PassthroughInline.Text() and removes all calls for Node.Text(), fixing the regression.

* passthrough: Add regression test

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

Add PassthroughBlock.Delimiters

deps: Upgrade github.com/yuin/goldmark v1.7.1 => v1.7.2

readme: Update extras extension example

github: Bump Go versions