Change the syntax of the ::open and :/close function calls

This is a follow-up to #364, which made it possible to use unquoted literals in the argument position in placeholders. However, due to the current syntax of +open and -close function calls, arguments that are number literals must still be quoted, e.g. `{|-1| :number}`.

This PR proposes to change the syntax of markup-like function calls:

    BEFORE: {+button title=|Click me!|}Submit{-button}
    AFTER:  {::button title=|Click me!|}Submit{:/button}

The benefit of using a two-char-long prefix is that we effectively establish the colon `:` as the general-purpose function introducer.

Author: stasm

spec/formatting.md

@@ -483,14 +483,14 @@ Between the brackets, the following contents are used:
   > Example: `{:platform}`
 
 - Opening _expression_ with no operand:
-  U+002B PLUS SIGN `+` followed by the _function_ _name_
+  U+003A COLON `:` followed by U+003A COLON `:` and then by the _function_ _name_
 
-  > Example: `{+tag}`
+  > Example: `{::tag}`
 
 - Closing _expression_ with no operand:
-  U+002D HYPHEN-MINUS `-` followed by the _function_ _name_
+  U+003A COLON `:` followed by U+002F SOLIDUS `/` and then by the _function_ _name_
 
-  > Example: `{-tag}`
+  > Example: `{:/tag}`
 
 - Otherwise: The U+FFFD REPLACEMENT CHARACTER `�` character
 

spec/message.abnf

@@ -14,7 +14,7 @@ annotation = (function *(s option)) / reserved
 
 literal = quoted / unquoted
 variable = "$" name
-function = (":" / "+" / "-") name
+function = ":" [":" / "/"] name
 option = name [s] "=" [s] (literal / variable)
 
 ; reserved keywords are always lowercase

spec/syntax.md

@@ -146,13 +146,13 @@ plucking the first name from the object representing a person:
 Functions use one of the following prefix sigils:
 
 - `:` for standalone content
-- `+` for starting or opening elements
-- `-` for ending or closing elements
+- `::` for starting or opening elements
+- `:/ for ending or closing elements
 
 A message with two markup-like _functions_, `button` and `link`,
 which the runtime can use to construct a document tree structure for a UI framework:
 
-    {{+button}Submit{-button} or {+link}cancel{-link}.}
+    {{::button}Submit{:/button} or {::link}cancel{:/link}.}
 
 An opening element MAY be present in a message without a corresponding closing element,
 and vice versa.
@@ -395,17 +395,17 @@ option = name [s] "=" [s] (literal / variable)
 > ```
 >
 > ```
-> {+ssml.emphasis level=strong}
+> {::ssml.emphasis level=strong}
 > ```
 >
 > Message examples:
 >
 > ```
-> {This is {+b}bold{-b}.}
+> {This is {::b}bold{:/b}.}
 > ```
 >
 > ```
-> {{+h1 name=above-and-beyond}Above And Beyond{-h1}}
+> {{::h1 name=above-and-beyond}Above And Beyond{:/h1}}
 > ```
 
 #### Reserved
@@ -483,7 +483,7 @@ unquoted-start = name-start / DIGIT / "."
 ### Names
 
 The **_name_** token is used for variable names (prefixed with `$`),
-function names (prefixed with `:`, `+` or `-`),
+function names (prefixed with `:`, `::` or `:/`),
 as well as option names.
 It is based on XML's [Name](https://www.w3.org/TR/xml/#NT-Name),
 with the restriction that it MUST NOT start with `:`,
@@ -492,7 +492,7 @@ Otherwise, the set of characters allowed in names is large.
 
 ```abnf
 variable = "$" name
-function = (":" / "+" / "-") name
+function = ":" [":" / "/"] name
 
 name = name-start *name-char
 name-start = ALPHA / "_"