Allow alloy fmt to show context

[petewall]

Request

When using alloy fmt to validate a config and there is a syntax value, sometimes the line number is not helpful enough to find the issue. This is because many times the config is composed from many config files. Here's how it looks today:

% alloy fmt config.alloy
config.alloy:5:23: expected TERMINATOR, got IDENT
Error: encountered errors during formatting

It would be great if it would provide a few lines of context to help show where the issue is. Something like:

% alloy fmt config.alloy
    targets    = discovery.relabel.cadvisor.output
    scheme     = "https"
>  scrape_interval = 10s
    bearer_token_file = "/var/run/secrets/kubernetes.io/serviceaccount/token"
    tls_config {
config.alloy:5:23: expected TERMINATOR, got IDENT
Error: encountered errors during formatting

Use case

It would really help show where errors are, especially with composed configurations.

[rfratto]

This is a good idea. I put it on our proposal queue because I think it needs some consideration for whether doing this by default might break existing editor integrations or make stderr harder to parse for tools.

Without putting more thought into whether a change could break things, my default stance would be to make this opt-in with a flag.