Re-architect format.py in swift

Make a couple of changes to `format.py` and use the opportunity to re-write it in Swift.

1. Always build a local copy of swift-format in `swift-syntax/.swift-format-build` in release mode. This eliminates issues that occur if you had `swift-format` installed on your system and `format.py` would pick it up
2. Since the local swift-format build is persistent (doesn’t live in `/tmp`), we can build it in release. This increases the first run but decreases the time for any subsequent runs from ~15s to ~1s, which should pay usually pay off for frequent contributors because of faster turnaround times.
3. Add a little tip to add `swift-format` as a git hook to Contributing.md

And as an added bonus, the rewrite reduced the runtime of `format.(py|swift)` from 1.2s to 800ms.

Author: ahoppen

.gitignore

@@ -20,3 +20,6 @@ Package.resolved
 *.pyc
 
 Tests/PerformanceTest/baselines.json
+
+# The local build of swift-format to format swift-syntax
+.swift-format-build

CONTRIBUTING.md

@@ -19,12 +19,22 @@ swift-syntax is a SwiftPM package, so you can build and test it using anything t
 
 swift-syntax is formatted using [swift-format](http://github.com/apple/swift-format) to ensure a consistent style.
 
-To format your changes run `format.py` at the root of this repository. If you have a `swift-format` executable ready, you can pass it to `format.py`. If you do not, `format.py` will build its own copy of `swift-format` in `/tmp/swift-format`.
-
-If you are seeing surprising formatting results, you most likely have a `swift-format` installed on your system that’s not the most recent version built from the `main` branch. To fix this, clone [swift-format](http://github.com/apple/swift-format), build it using `swift build` and pass the freshly built executable to `format.py` as `--swift-format path/to/swift-format/.build/debug/swift-format`. Alternatively, you can uninstall `swift-format` on your system and `format.py` will build it from scratch.
+To format your changes run `format.swift` at the root of this repository from the terminal as `./format.swift`. It will build a local copy of swift-format from the `main` branch and format the repository. Since it is building a release version of `swift-format`, the first run will take few minutes. Subsequent runs take less than 2 seconds.
 
 Generated source code is not formatted to make it easier to spot changes when re-running code generation.
 
+> 💡 Tip: You can add a git hook to ensure all commits to the swift-syntax repository are correctly formatted. 
+> 1. Save the following contents to `swift-syntax/.git/hooks/pre-commit`
+> ```bash
+> #!/bin/bash
+> set -e
+> SOURCE_DIR=$(realpath "$(dirname $0)/../..")
+> "$SOURCE_DIR/format.swift" --lint
+> ```
+> 2. Mark the file as executable: `chmod a+x swift-syntax/.git/hooks/pre-commit`
+> 3. If you have any global git hooks installed, be sure to call them at the end of the script: `path/to/global/hooks/pre-commit "$@" 
+
+
 ## Generating Source Code
 
 If you want to modify the generated files, open the [CodeGeneration](CodeGeneration) package and run the `generate-swift-syntax` executable.