Proposal: Improve Watch Mode for a better development experience

[stephenafamo]

Continuation of #318

The problem

When working with Templ, the time between making a change and being able to view the change is often too slow.
This is because the Go code has to be generated from the template, and then the whole application has to be recompiled and restarted.

Having the whole thing recompiled is good for production since it would execute quickly, but during development, it is a pain to wait for restarts for every little change.

Proposed solution

I would suggest the following changes to --watch

On startup

1. The template is parsed and split into "static strings" and "go code".
2. A hash of the "go code" is calculated and stored in memory.
3. The static strings are put in a separate file where they are read from during template execution.

On change

1. The hash of the go code is compared to the previous hash. If the go code changed, the _templ.go file is regenerated.
2. The static strings are updated.

On end

1. The _templ.go file is regenerated with the static strings written directly into the file, the same way it currently works.
2. The file with the static strings is deleted.

Edit

Implemented in #366