WIP documentation

- The documentation is cluttered enough as it is with the large number
  of procedures supporting vCard 3 and 4. Split common out into the
  publicly exposed bits and the private internals. This makes it obvious
  which common functionality a client can expect to have exposed on the
  main vcard module.

- Add documentation (WIP) on the vcard3 module.

src/vcard/private/lexer.nim

@@ -1,9 +1,9 @@
-# VCard-specific Lexer
+# vCard-specific Lexer
 # © 2022-2023 Jonathan Bernard
 
-## This module defines a lexer with functionality useful for parsing VCard
+## This module defines a lexer with functionality useful for parsing vCard
 ## content. Specifically:
-## - it understands the VCard line-folding logic and transparently joins folded
+## - it understands the vCard line-folding logic and transparently joins folded
 ##   lines as it read input off its input stream.
 ## - it supports multiple nested bookmarks to make look-ahead decisions more
 ##   convenient