Introduction of ABNF for expressions embedded in strings

[char0n]

Hi,

I'm proposing to include and ABNF grammar to clearly determine how expressions embedded in strings are extracted.

; Grammar for parsing template strings with embedded expressions
expression-string    = *( literal-char / embedded-expression )
embedded-expression  = "{" expression "}"
literal-char         = %x00-7A / %x7C / %x7E-10FFFF  ; anything except { (%x7B) and } (%x7D)

There is still an issue related to JSON Pointers documented in #424.