message: Add unicode production
diff: The _lexical structure_ of the Pikelet programming langues is a description of what constitutes a valid sequence of tokens in the programming language.
  -## Whitespace and comments
  +## Unicode
  > **Grammar**:
  >
  +> <a href="#var:any-unicode-scalar"><var id="var:any-unicode-scalar">any-unicode-scalar</var></a> ::=\
  +> &emsp;|&ensp;<kbd>U+0000</kbd>&hellip;<kbd>U+7FFF</kbd>
  +>
  > <a href="#var:horizontal-tab"><var id="var:horizontal-tab">horizontal-tab</var></a> ::=\
  > &emsp;|&ensp;<kbd>U+0009</kbd>
  >
  @@ -35,6 +38,10 @@ The _lexical structure_ of the Pikelet programming langues is a description of w
  >
  > <a href="#var:paragraph-separator"><var id="var:paragraph-separator">paragraph-separator</var></a> ::=\
  > &emsp;|&ensp;<kbd>U+2029</kbd>
  +
  +## Whitespace and comments
  +
  +> **Grammar**:
  >
  > <a href="#var:line-break"><var id="var:line-break">line-break</var></a> ::=\
  > &emsp;|&ensp;<a href="#var:line-feed"><var>line-feed</var></a>\
  @@ -42,7 +49,7 @@ The _lexical structure_ of the Pikelet programming langues is a description of w
  > &emsp;|&ensp;<a href="#var:carriage-return"><var>carriage-return</var></a> <a href="#var:line-feed"><var>line-feed</var></a>
  >
  > <a href="#var:comment-text"><var id="var:comment-text">comment-text</var></a> ::=\
  -> &emsp;|&ensp;~(<a href="#var:line-feed"><var>line-feed</var></a> | <a href="#var:carriage-return"><var>carriage-return</var></a>) Any Unicode scalar value
  +> &emsp;|&ensp;~(<a href="#var:line-feed"><var>line-feed</var></a> | <a href="#var:carriage-return"><var>carriage-return</var></a>) <a href="#var:any-unicode-scalar"><var>any-unicode-scalar</var></a>
  >
  > <a href="#var:comment"><var id="var:comment">comment</var></a> ::=\
  > &emsp;|&ensp;`--` <a href="comment-text"><var>comment-text</var></a> <a href="#var:line-break"><var>line-break</var></a>
  @@ -103,10 +110,10 @@ The _lexical structure_ of the Pikelet programming langues is a description of w
  > **Grammar**:
  >
  > <a href="#var:character-literal"><var id="var:character-literal">character-literal</var></a> ::=\
  -> &emsp;|&ensp;`"` (`\"` | ~`"` Any Unicode scalar value)<sup>*</sup> `"`
  +> &emsp;|&ensp;`"` (`\"` | ~`"` <a href="#var:any-unicode-scalar"><var>any-unicode-scalar</var></a>)<sup>*</sup> `"`
  >
  > <a href="#var:string-literal"><var id="var:string-literal">string-literal</var></a> ::=\
  -> &emsp;|&ensp;`'` (`\'` | ~`'` Any Unicode scalar value)<sup>*</sup> `'`
  +> &emsp;|&ensp;`'` (`\'` | ~`'` <a href="#var:any-unicode-scalar"><var>any-unicode-scalar</var></a>)<sup>*</sup> `'`
  ### Tokens
  