scribble: decode a lone ` as a left "curly quote"

Also, add 'lsquo as allowed content.

Omitting the ` conversion in the first place was over-conservative.
There's a backward-compatibility issue with this addition (i.e., a
document might contain a backquote in a decoded context that is
meant to be rendered as a backquote), but the potential problems
seem minor.

collects/scribble/core.rkt

@@ -107,6 +107,7 @@
           [ldquo . #t]
           [rdquo . #t]
           [rsquo . #t]
+          [lsquo . #t]
           [prime . #t]
           [rarr . #t]
           [larr . #t]

collects/scribble/decode.rkt

@@ -105,7 +105,7 @@
 
 
 (define (decode-string s)
-  (define pattern #rx"(---|--|``|''|')")
+  (define pattern #rx"(---|--|``|''|'|`)")
   (let loop ([start 0])
     (cond
      [(regexp-match-positions pattern s start)
@@ -117,7 +117,8 @@
                    [(string=? the-match "--") 'ndash]
                    [(string=? the-match "``") 'ldquo]
                    [(string=? the-match "''") 'rdquo]
-                   [(string=? the-match "'") 'rsquo])
+                   [(string=? the-match "'") 'rsquo]
+                   [(string=? the-match "`") 'lsquo])
                   (loop (cdar m))))]
      ;; Common case: nothing to decode, so don't copy strings.
      ;; Assume that the input is already interned.

collects/scribble/latex-render.rkt

@@ -729,6 +729,7 @@
                     [(ldquo) "{``}"]
                     [(rdquo) "{''}"]
                     [(rsquo) "{'}"]
+                    [(lsquo) "{`}"]
                     [(prime) "$'$"]
                     [(rarr) "$\\rightarrow$"]
                     [(larr) "$\\leftarrow$"]

collects/scribble/text-render.rkt

@@ -162,6 +162,7 @@
                     [(ndash) "\U2013"]
                     [(ldquo) "\U201C"]
                     [(rdquo) "\U201D"]
+                    [(lsquo) "\U2018"]
                     [(rsquo) "\U2019"]
                     [(lang) ">"]
                     [(rang) "<"]

collects/scribblings/scribble/core.scrbl

@@ -137,7 +137,7 @@ A @deftech{block} is either a @techlink{table}, an
 
                    @item{A symbol content is either @racket['mdash],
                          @racket['ndash], @racket['ldquo],
-                         @racket['lsquo], @racket['rsquo], @racket['larr],
+                         @racket['lsquo], @racket['rdquo], @racket['rsquo], @racket['larr],
                          @racket['rarr], or @racket['prime]; it is
                          rendered as the corresponding HTML entity
                          (even for Latex output).}

collects/scribblings/scribble/decode.scrbl

@@ -28,6 +28,8 @@ makes just a few special text conversions:
 
  @item{@litchar{'}: converted to @racket['rsquo], which is a fancy apostrophe: '}
 
+ @item{@litchar{`}: converted to @racket['lsquo], which is a fancy quote: `}
+
 ]
 
 Some functions @deftech{decode} a sequence of @racket[_pre-flow] or

collects/tests/scribble/docs/decode.scrbl

@@ -0,0 +1,4 @@
+#lang scribble/base
+
+``To make your so--so document look `pretty,' use fancy quotes,'' he
+said---with practically audible quotes.

collects/tests/scribble/docs/decode.txt

@@ -0,0 +1,2 @@
+“To make your so–so document look ‘pretty,’ use fancy quotes,” he
+said—with practically audible quotes.