PackageToJS: Inherit swift package -c configuration by default

[kateinoigakukun]

#309 introduced -c option to the plugin side in addition to the swift package's one. However swift package -c release js was building with release config before 0.25.0 but it started to build with debug config as the default in 0.25.0. This change makes the plugin to respect the swift package's one if -c is not specified after js plugin name

[github-actions]

Time Change: +239ms (2%)

Total Time: 9,855ms

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import with int	24ms	+3ms (12%)	⚠️
Serialization/Swift Int to JavaScript with assignment	349ms	+24ms (6%)	🔍

View Unchanged

Test name	Duration	Change
Serialization/JavaScript function call through Wasm import	25ms	+1ms
Serialization/JavaScript function call from Swift	132ms	+5ms (3%)
Serialization/Swift Int to JavaScript with call	997ms	+43ms (4%)
Serialization/JavaScript Number to Swift Int	272ms	+12ms (4%)
Serialization/Swift String to JavaScript with assignment	427ms	+21ms (4%)
Serialization/Swift String to JavaScript with call	1,070ms	+32ms (3%)
Serialization/JavaScript String to Swift String	3,725ms	+105ms (2%)
Object heap/Increment and decrement RC	2,798ms	-11ms (0%)

View Baselines

Test name	Duration
Serialization/Call JavaScript function directly	7ms
Serialization/Assign JavaScript number directly	9ms
Serialization/Call with JavaScript number directly	7ms
Serialization/Write JavaScript string directly	7ms
Serialization/Call with JavaScript string directly	6ms