userdiff: add support for Swift

userdiff: add support for Swift

Add a built-in userdiff driver for the Swift programming language so that
diff hunk headers and word diffs work out of the box for ".swift" files.

The funcname pattern is built for Swift's own declaration grammar: an
optional run of attributes ("@objc", "@available(iOS 13, *)", ...),
followed by an optional run of lowercase modifiers ("public", "static",
"final", ...), followed by a declaration keyword (func, class, struct,
enum, protocol, extension, actor, init, deinit, subscript). The keyword
is followed by a boundary that allows whitespace, "(" (init/subscript),
"?" or "!" (failable init), or "<" (generics), while still acting as a
word boundary so e.g. "initialize(" does not match.

The word regex recognizes Swift identifiers, hexadecimal, octal, binary,
integer and floating-point literals, and the language's operators.

Signed-off-by: Shlok Kulshreshtha <redacted>
Acked-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>