Support boolean strings in environment variables

[timeverts]

Description

Currently, setting an environment variable value in .env to true or false string literals and fetching that using Craft::parseEnv() does not return the boolean equivalent value, as might be expected by many developers. Instead the string value 'true' or 'false' is returned, so boolean comparisons on the parsed environment variable always equate to true.

Some other popular frameworks natively implement this string to boolean conversion functionality in their environment parsing methods. For example, Laravel's environment parsing method does the following:

    /**
     * Gets the value of an environment variable. Supports boolean, empty and null.
     *
     * @param  string  $key
     * @param  mixed   $default
     * @return mixed
     */
    function env($key, $default = null)
    {
        $value = getenv($key);
        if ($value === false) {
            return value($default);
        }
        switch (strtolower($value)) {
            case 'true':
            case '(true)':
                return true;
            case 'false':
            case '(false)':
                return false;
            case 'empty':
            case '(empty)':
                return '';
            case 'null':
            case '(null)':
                return;
        }
        if (Str::startsWith($value, '"') && Str::endsWith($value, '"')) {
            return substr($value, 1, -1);
        }
        return $value;
    }

It would be great if Craft::parseEnv() could be made more intelligent to handle boolean type environment variables and alias values.

FYI: There is an interesting discussion about the support of boolean environment variables on the official phpdotenv github.

Steps to reproduce

1. Define an environment variable in .env with a false string value (e.g. TEST_MODE=false)
2. Parse it: $isTestMode = Craft::parseEnv('TEST_MODE')
3. Test it: echo ($isTestMode ? 'In Test Mode' : 'Not In Test Mode'); outputs In Test Mode instead of Not In Test Mode.

Additional info

• Craft version: 3.1.17.1
• PHP version: 7.1
• Database driver & version: mySQL 5.7.14
• Plugins & versions: N/A