ECMAScript

As Clippy warns, the RegExp structure in src/builtins/regexp/mod.rs has too many booleans (one for each flag). This is bad, because it might not get correctly optimised by the compiler. Ideally, we would want to use a bitflags structure such as the RegExpFlags one in https://github.com/boa-dev/boa/blob/60b7eb89340c2

#1263

Todoのlistにulが抜けているためスタイルがおかしくなっている

I will create a new project for this epic and several tickets related to it.

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