Support for std::string_view in proxy_base

[misyltoad]

Allows code like the following to work:

std::string_view pretty_name = my_table["pretty_name"];

Otherwise it fails with:

error: conversion from ‘sol::table_proxy<sol::basic_table_core<false, sol::basic_reference<false> >&, std::tuple<const char (&)[12]> >’ to non-scalar type ‘std::string_view’ {aka ‘std::basic_string_view<char>’} requested
 2131 |                         std::string_view psvPrettyName = tTable["pretty_name"];
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