Make actix dependency optional

[kamoshi]

Currently, when using the pagefind crate as a library inside another Rust project, the crate pulls in ~340 dependencies, including actix, even when the "serve" functionality is not being used at all inside the project.

I think it would be beneficial to make the actix dependency (there might me more unneeded, but haven't checked anything else) optional and put the "serve" functionality behind a compile flag.

This can be done as follows (example):

In Cargo.toml

# ...

[features]
default = ["serve"]
serve = ["dep:actix-web", "dep:actix-files", "dep:portpicker"]

# ...

[dependencies]
# ... 
actix-web = { version = "4", optional = true }
actix-files = { version = "0.6", optional = true }
portpicker = { version = "0.1", optional = true }

In *.rs:

// ...
#[cfg(feature = "serve")] // +
mod serve;
// ...

I think it would be generally beneficial to try to trim down the binary size for use cases that don't require as many features as a standalone binary program. This serve feature seems like a low-hanging fruit to me, because it cuts down dependencies from 342 to 214.

Full git diff output: diff.txt

[bglw]

Yep! This makes total sense, I haven't looked over the deps since publishing the crate for use. Will do.

[bglw]

Done in #1023 so this will make the next stable release.

I canvassed for any others, the most obvious being the CLI/config parsing libs (twelf + clap etc), but that will need a bit more reshaping of the config structs.