Make SQLiteVersion struct sendable

[jozefizso]

To add support for Swift 6 concurrency model in the SQLite.swift package, the SQLiteVersion struct must be made sendable.

The static variable SQLiteVersion.zero must be changed to a immutable let variable:

-    static var zero: SQLiteVersion = .init(major: 0, minor: 0)
+    static let zero: SQLiteVersion = .init(major: 0, minor: 0)

This will be a breaking change for clients who change the value of SQLiteVersion.zero.point as the point field is defined as public var:

public struct SQLiteVersion: Comparable, CustomStringConvertible {
    public let major: Int
    public let minor: Int
    public var point: Int = 0
}

I think it is not desired to change the value of the SQLiteVersion.zero and it can be safely changed to let.

[jozefizso]

I can make the PR for this change.