Introduce One

[rxwei]

One is a lightweight component that allows the use of the leading dot syntax to reference RegexComponent static members such as character classes as a non-first expression in a regex builder block.

---

Before:

Regex {
    .digit // works today but brittle; inserting anything above this line will break this

    OneOrMore(.whitespace)

    .word // ❌ error: 'OneOrMore' has no member named 'word' (because this is parsed as a member reference on the preceeding expression)
}

After:

Regex {
    One(.digit)              // recommended even though `.digit` works today
    OneOrMore(.whitespace)
    One(.word)
} // ✅

In a follow-up patch, we will propose adding an additional protocol inheriting from RegexComponent that will ban the use of the leading dot syntax even on the first line of Regex { ... }, as this will enforce the recommended style (use of One), and prevent surprises when the user inserts a pattern above the leading dot line.

[rxwei]

@swift-ci please test