Fix LazyString building with non-ASCII preceding $

[Liozou]

Simple bugfix for building LazyString with an $ sign just after a character of encoding longer than one codeunit.

master:

julia> lazy"foo n°$3"
ERROR: LoadError: StringIndexError: invalid index [7], valid nearby indices [6]=>'°', [8]=>'$'
Stacktrace:
 [1] string_index_err(s::String, i::Int64)
   @ Base ./strings/string.jl:12
 [2] getindex(s::String, r::UnitRange{Int64})
   @ Base ./strings/string.jl:279
 [3] var"@lazy_str"(__source__::LineNumberNode, __module__::Module, text::Any)
   @ Base ./strings/lazy.jl:70
in expression starting at REPL[8]:1

This PR:

julia> lazy"foo n°$3"
"foo n°3"