Adds Swift version support

[antoniostrijdom]

This PR adds the ability to specify the Swift versions your package supports as well as the ability to specify which swift-tools-version to use. These properties are entirely optional.

In order to specify which versions of Swift your module supports, specify it in the new swiftLanguageVersions array in your package definition:

{
  "name": "Example",
  ...
  "swiftLanguageVersions": [
    "5.10",
    "6.0"
  ],
  ...
}

this will produce a package that supports Swift 5.10 and Swift 6:

swiftLanguageVersions: [
    .version("5.10"),
    .version("6.0"),
]

In order to specify which swift-tools-version to use, specify it in the new swiftToolsVersion property in your package definition:

{
  "name": "Example",
  ...
  "swiftToolsVersion": "5.10",
  ...
}

this will product a package that uses 5.10 as the swift-tools-version:

// swift-tools-version: 5.10

[albertodebortoli]

Instead of isToolsVersion I would prefer a dedicated key in the spec. What do you think?

Minor: use 2 spaces in the jsons for consistency.