Berry f-strings now support '::' in expression

[s-hadinger]

Description:

Berry, now f-strings supports ':' in expression, if double as '::'.

Example:

var is_on = true
var s = f"status = {is_on ? 'on' :: 'off'}"
# s is 'status = on'

f"status = {is_on ? 'on' :: 'off'}" is equivalent to format("status = %s", is_on ? 'on' : 'off')