Fix within string subset completions on non-syntactic names

[DavisVaughan]

Closes posit-dev/positron#14909

Changed from enquote: bool to

pub(super) enum QuoteStyle {
    // Double quote with `"`
    Double,

    // Backtick with `\`` if the symbol isn't syntactic
    BacktickIfNotSyntactic,

    // No quoting. Useful if you know you're already inserting into an existing string.
    None,
}

It was a bit weird before, but the previous mapping was:

• enquote: true -> QuoteStyle::Double
• enquote: false -> QuoteStyle::BacktickIfNotSyntactic

So we previously had no way to say "please just take this as is, I know what I'm doing", which is the case of x["<TAB>"] where we are completing within an existing string and we don't want any quoting whatsoever.