Major updates to Caddyfile lexer

[francislavoie]

Since I've last touched the lexer in #363 #365 we've had plenty of minor syntax adjustments to the Caddyfile. Took me a long time to get in the headspace to want to tackle updating the lexer, it's quite complicated!

We use this lexer for the official Caddy docs https://caddyserver.com/docs/caddyfile/concepts where Caddy itself serves its own syntax via Markdown. It's awesome that Caddy can highlight its own code 😁

I went through every page in the docs we have to find all the edgecases that were broken in one way or another. I used a go.mod replace in Caddy to point to my checkout of Chroma which I was editing, and used https://github.com/cespare/reflex to watch for my file changes and rebuild+restart Caddy so I could just refresh my browser to see effect of my changes to the lexer. I used this crazy command to do this (had to cross through 3 different git repos with cd 😂)

$ (cd ../../../chroma/ && reflex -s -g 'lexers/caddyfile.go' -- sh -c 'cd ../caddy/cmd/caddy && go build && cd ../../../website/ && ../caddy/cmd/caddy/caddy run')

Added lots more contrived config examples to the caddyfile.actual test to cover most of the changes I've made.

I'm not satisfied with the heredoc (see #929) because I'd like to be able to parse the placeholders (LiteralStringEscape) inside the heredoc text, but I'd probably need to write a custom Mutator to do that and that felt daunting. But good enough for now I think.

[francislavoie]

lexers/caddyfile.go:11:5: G101: Potential hardcoded credentials (gosec)

😂 what the heck, I guess I'll throw a // nolint on that

[alecthomas]

Go home linter, you're drunk.