Change default ruby parser to RDoc::Parser::PrismRuby

[tompng]

Fixes #398, #782, #816, #1555, #885, #1373, #1553

Although generated HTML has huge difference, many many bugs are fixed with RDoc::Parser::PrismRuby parser.
Diffs are mostly: many bugfixes, crossrefs, trailing whitespaces in syntax highlight, and perhaps few new bugs.
Old parser can be used with RDOC_USE_RIPPER_PARSER=1

[matzbot]

🚀 Preview deployment available at: https://39dda6de.rdoc-6cd.pages.dev (commit: 2445ddc)

[tompng]

RDoc::Parser::Ruby specially handles :call-seq:.

# This is a call-seq
#   :call-seq: foo(x)
#
# This is a code block in RDoc::Parser::Ruby,
# but a second call-seq in RDoc::Prism::PrismRuby
#   :call-seq: foo(y)
#
# Code block in both parser
#   \:call-seq: foo(z)

def foo(*); end

Indented directives are allowed because indented :include: have special meaning.

# include a file with all lines indented
#   :include: file.rb