proposed symbol list utility function

### Description

symbol cleanup utility function i use. i can turn this into a pr if you think its worth moving into quantmod

```r
cleanSymbolList <- function(Symbols, to.upper = F, filter.na = T) {
importDefaults("cleanSymbolList")
if (is.null(Symbols)) return(NULL)
if (length(Symbols) == 0) return(character())

if (is.list(Symbols)) Symbols <- unlist(Symbols)
if (!is.character(Symbols)) stop( "cleanSymbolList only accepts chracter input")

Symbols <- unique(trimws(unlist(strsplit(Symbols, ";"))))

if (to.upper) Symbols <- toupper(Symbols)
if (filter.na) Symbols <- Symbols[!is.na(Symbols)]
return(Symbols)
}
```

No file or test is named. Start by reviewing the proposed cleanSymbolList function and quantmod's existing utility entry points to determine where such a function would belong; done means its proposed behavior and integration point have been agreed and verified for the inputs shown.

Written by the indexing model from the issue text.