Add support for string literal concatenation

[etgarperets]

… space in mySQL

[iffyio]

Suggested change

	// see <https://dev.mysql.com/doc/refman/8.4/en/string-functions.html#:~:text=mysql%3E%20SELECT%20%27My%27%20%27S%27%20%27QL%27%3B%0A%20%20%20%20%20%20%20%20%2D%3E%20%27MySQL%27>
	/// see <https://dev.mysql.com/doc/refman/8.4/en/string-functions.html#function_concat>

[iffyio]

Suggested change

	// Does the Dialect support concatenating of string literal
	// Example: SELECT 'Hello ' "world" => SELECT 'Hello world'
	fn supports_concat_quoted_identifiers(&self) -> bool {
	/// Does the Dialect support concatenating of string literal
	/// Example: `SELECT 'Hello ' "world" => 'Hello world'`
	fn supports_string_literal_concatenation(&self) -> bool {

[iffyio]

Can we simplify with something roughly like the following?

fn maybe_concat_string_literal<F: FnOnce(String) -> Value>(
	mut str: String,
	value_fn: F
) -> Result<Value> {
	if self.dialect.supports_concat_quoted_identifiers() {
		loop {
			match self.peek_token_ref() {
				Token::SingleQuotedString(s) | Token::DoubleQuotedString(s) => {
					self.advance_token();
					str.append(s);
				}
				_ => {
					break
				}
			}
		}
	}

	Ok(value_fn(str))
}

And the caller only does e.g.

Token::SingleQuotedString(ref s) => self.maybe_concat_string_literal(s.to_string(), Value::SingleQuotedString)

[iffyio]

since this is gated by a dialect flag can we use all_dialects_where(|d| d.supports...) so that new dialects are covered by tests when enabled