Add clean if-file-deleted setting - #90

Adds a new option for the clean setting called if-file-deleted. In this mode, we clean output files if any of the input files have been deleted since the previous run.

This is useful for commands like tsc --build:

• "clean": true (the default) is not a good option for TypeScript, because it either eliminates the benefits of incremental compilation, or causes your .tsbuildinfo to get out of sync, depending on whether you include your .tsbuildinfo file in the output array.

• "clean": false is also not a good option for TypeScript, because it causes stale outputs to accumulate. This is because when you delete or rename a .ts source file, tsc itself does not automatically delete the corresponding .js file emitted by previous compiles.

• "clean": "if-file-deleted" is a nice balance between fast and correct output. It lets you use tsc fast incremental compiles when a .ts source file is added or modified, but a clean build when a .ts source file is deleted.

Example

{
  "scripts": {
    "ts": "wireit",
  },
  "wireit": {
    "ts": {
      "command": "tsc --build --pretty",
      "clean": "if-file-deleted",
      "files": [
        "src/**/*.ts",
        "tsconfig.json"
      ],
      "output": [
        "lib/**",
        ".tsbuildinfo"
      ]
    }
  }

Fixes #70

Also

• Renames CacheKey to ScriptState. I think this is a more clear name, because this data type is used for purposes other than caching (e.g. checking freshness, and now also for comparing input file names). It also maps more closely to the existing .wireit/<script>/state file, which is where we serialize state.

• Some refactoring to ScriptState. We previously did not save any ScriptState at all when a script was uncacheable, and used a special UNCACHEABLE sentinel to represent that case. However, this meant that we had no record of the previous input files in the case where a script was uncacheable, which we now need. We now instead always write a state file, and set an uncacheable property on it.

• Added a "Recipes" section to the README, for tool-specific guidance. Starting with TypeScript, since that was the motivation for this change.