v4.0.0

Aligns the library with v2 of the llms.txt specification. Closes issue 10.

Added

• New optional and getOptional methods to set and access the Optional section of a llms.txt file.
• New Discovery class for the link relations v2 introduced. It renders rel="alternate" type="text/markdown" and
  rel="describedby" as HTML <link> elements or as an HTTP Link header value, reads both back out of HTML content
  or a Link header value, and provides the specification's Markdown URL forms and subpath scoping as helpers.
• New ValidationResult::warnings and ValidationResult::hasWarnings methods, reporting the recommended, but not
  specification required, elements a llms.txt file are missing.

Changed

• The Optional section is no longer skipped when expanding a llms.txt file into a LLM context file,
  since v2 removed its mechanical semantics along with the llms_txt2ctx tooling. The $includeOptional parameter of
  toLlmContext, toLlmContextFile, and LlmContext::expand has been replaced by $skipOptional, which inverts the
  meaning of a passed argument.
• validate now reports a missing title as the only validation error, as it is the only element the
  specification requires. A missing description, details, section, or section link is reported as a validation warning
  instead of an error, which makes validate return true for files it rejected before.
• validate now looks for section links in all sections instead of only in the first one.