Revert reservation of ":" within styled regions

In 5dd8b9e we reserved the use of ":" in styled regions in
anticipation of the potential use of a style and format construction
"{style:content:format}" inspired by Python's f-strings.

It has since become apparent that this causes more headaches than
anticipated in Julia. The primary reason for this is the different
syntactic meaning that ":" has in Julia compared to Python. While in
Python ":" serves as a block terminator (used after: if, for, while,
def, etc.), in Julia ":" can appear /within/ expressions (namely with
ranges and indexing), for example:

  1:2:9
  m[1, :]
  :(1 + 2)

This means that when formatting an expression, copy-paste is now subtly
error-prone as not just quotes but colons will need to be escaped.

This adds to the minor, but present, annoyance of having to escape ":"
in "{style:content}" constructions, and in my mind tips the scales
towards the prospective "{style:content:fmt}" syntax no longer being
worth it.

A more promising alternative is to go with something like "${fmt:expr}".
By both this and "{style:content}" having a single delimiting colon, we
can simply read everything until the paired closing curly parentheses at
the end of the form as expr/content. The use of "$" in "${fmt:expr}" is
also evocative of Julia's $-interpolation syntax, which is appropriate
given that expr would be evaluated.

While a formating syntax, let alone system, is not decided on at this
stage, I think we can safely un-reserve the colon.

Author: tecosaur

src/stylemacro.jl

@@ -36,9 +36,7 @@ escaped = '\\\\', specialchar ;
 interpolated = '\$', ? expr ? | '\$(', ? expr ?, ')' ;
 
 styled = '{', ws, annotations, ':', content, '}' ;
-content = { interpolated | colonescaped | almostplain | styled } ;
-colonescaped = escaped | '\\\\:' ;
-almostplain = { plain - ':' } ;
+content = { interpolated | plain | escaped | styled } ;
 annotations = annotation | annotations, ws, ',', ws, annotation ;
 annotation = face | inlineface | keyvalue ;
 ws = { ' ' | '\\t' | '\\n' } ; (* whitespace *)
@@ -109,7 +107,7 @@ macro styled_str(raw_content::String)
     end =#
 
     # Instead we'll just use a `NamedTuple`
-    state = let content = unescape_string(raw_content, ('{', '}', ':', '$', '\n', '\r'))
+    state = let content = unescape_string(raw_content, ('{', '}', '$', '\n', '\r'))
          (; content, bytes = Vector{UInt8}(content),
          s = Iterators.Stateful(pairs(content)),
          parts = Any[],
@@ -208,7 +206,7 @@ macro styled_str(raw_content::String)
     end
 
     function escaped!(state, i, char)
-        if char in ('{', '}', ':', '$', '\\')
+        if char in ('{', '}', '$', '\\')
             deleteat!(state.bytes, i + state.offset[] - 1)
             state.offset[] -= ncodeunits('\\')
         elseif char ∈ ('\n', '\r')
@@ -651,8 +649,6 @@ macro styled_str(raw_content::String)
                 else
                     styerr!(state, "Contains extraneous style terminations", -2, "right here")
                 end
-            elseif char == ':' && !isempty(state.active_styles)
-                styerr!(state, "Colons within styled regions need to be escaped", -2, "right here")
             end
         end
         # Ensure that any trailing unstyled content is added