Add asciidoc formatter for AsciiDoc files

[dheid]

Adds a pure-Java AsciidocFormatterStep to lib (no external dependencies) and wires it into both the Gradle and Maven plugins as a first-class asciidoc format type.

What it does

The formatter applies a configurable pipeline of structural normalizations to .adoc files. Every option is a boolean flag; all default values are chosen to be safe on real-world documents.

Option	Default	Effect
normalizeSetextHeadings	true	Convert underline-style headings (Title\n=====) to ATX style (= Title)
removeTrailingHeaderEqualsSign	true	Remove symmetric trailing = signs (== Title == → == Title) and normalize whitespace after
the leading = signs
ensureHeadingBlankLines	true	Insert blank lines before and after every section heading
normalizeBlockDelimiters	true	Shorten over-long delimiters (-------- → ----)
collapseConsecutiveBlankLines	true	Collapse multiple consecutive blank lines into one
removeTrailingWhitespace	true	Strip trailing whitespace from every line
oneSentencePerLine	true	Reflow paragraph text so each sentence starts on its own line
titleCase	false	Apply Chicago-style title case to section headings and block titles
normalizeListBullets	false	Convert dash bullets to asterisk style (- item → * item)
normalizeOrderedListMarkers	false	Convert explicit numbers to AsciiDoc dot style (1. item → . item)
ensureSourceDelimiters	false	Wrap bare [source,…] / [listing] blocks that have no ---- delimiter

All transformations respect delimited block boundaries — content inside ----,
====, ****, etc. is never modified.

Usage

Gradle:

spotless {
  asciidoc {
    target '**/*.adoc'
    asciidoc()
      // override any defaults here, e.g.:
      .titleCase(true)
      .normalizeListBullets(true)
  }
}

Maven:

<asciidoc>
  <includes><include>**/*.adoc</include></includes>
  <asciidocFormatting>
    <titleCase>true</titleCase>
    <normalizeListBullets>true</normalizeListBullets>
  </asciidocFormatting>
</asciidoc>

Tests

• 122 unit tests in lib (AsciidocFormatterFuncTest) covering every option individually, including idempotency, block-boundary protection, and several bug-regression cases (setext detection inside code blocks, page-break / block-macro handling, tab-separated list markers).
• AsciidocFormatterStepTest in testlib using StepHarness.testResource for an end-to-end before/after fixture, and SerializableEqualityTester to verify that every config field contributes to step identity.

Notes

• The Maven plugin defaults are more conservative than the lib defaults (only removeTrailingWhitespace, collapseConsecutiveBlankLines, and ensureHeadingBlankLines are on) to avoid surprising users who opt in
  without reading the docs.
• The oneSentencePerLine feature handles common AsciiDoc-specific structural lines (page breaks <<<, block macros include::, toc::, description-list terms, horizontal rules ''') so they are never accumulated into paragraph text.
• All new files carry the standard Apache 2.0 / Copyright 2026 DiffPlug header. spotlessApply and spotbugsMain both pass.

[dheid]

@nedtwigg Could you review this pull request plz?