Improve performance

[or]

As most users, I use linters and formatters mainly in my editor, but also to check or fix a larger code base at the command line or in workflows.
For both of those usages performance is a factor, so I've been trying to reduce cljfmt start-up and run time.

• I've experimented with using pmap to process multiple files for fix, and that sped up fixing a largeish directory by a factor of 2-3, so I think a --parallel option would be useful, would you consider a PR for that feature?
• the plugin to create a native image currently has a bug that prevents the :jvm-opts specified for it to be used (see taylorwood/lein-native-image#8), if those flags are used, then the executable is about 20% faster on my machine (with GraalVM CE or EE); I only mention this here as a heads up, as I hope the plugin will be fixed soon, but if that takes longer than expected it might be worth to use the workaround
• a small performance improvement for native-image is not using read-string here: https://github.com/or/cljfmt/blob/da37b7aef08be99dbb8e2e8772d95308150d7adb/cljfmt/src/cljfmt/main.clj#L146, which is done to allow .clj files for indents and aliases; a little surprisingly this doubles the size of the native-image executable and removing it improves performance by about 4-5% - I realize that's not huge, but would you consider a build flag that turns off .clj support?

The largest improvement is using GraalVM EE, which has nothing to do with cljfmt, but it almost doubles the speed out of the box, and even there the jvm-opts and clj-support removal have a noticeable impact. For the largest CLJS file in my project the current master took a median of 556ms to fix an already formatted file (with the various options I use), with the changes it's down to 443ms. With GraalVM EE this crumbles to 258ms.
These are all acceptable numbers, but I think it's worth optimizing this as much as possible, so it becomes unnoticeable when used in an on-save handler in the editor.

For reference:

version	GraalVM	size in MB	–help	check in ms	fix in ms	speed %	EE speed %
master	CE 22.1.0 Java 17	37	91	568	556	100
jvm-opts	CE 22.1.0 Java 17	36	85	473	464	120
jvm-opts, no-clj	CE 22.1.0 Java 17	17	80	451	443	126
master	EE 22.1.0 Java 17	36	61	320	314	177	100
jvm-opts	EE 22.1.0 Java 17	37	56	277	267	208	118
jvm-opts, no-clj	EE 22.1.0 Java 17	19	54	265	258	216	122

I haven't tried a comparison with the leiningen plugin, but perhaps a section in the README on these options (lein, GraalVM CE, and EE) and how they compare could be useful for anyone wanting to adopt cljfmt and having performance concerns.

[weavejester]

Sure, I'll accept any well written PR that improves performance.