feat: fastbelt scaffold

[SteveRuble]

This implements a fastbelt scaffold command.

It can produce an entire Go module or just add a package to an existing module.

The generated code includes the VS Code extension, LSP server, and go:generate wiring, as well as a placeholder .fb file.

Partially completes #57 (the scaffolding part).

This is a big PR but that's inevitable when you're introducing scaffolding... there are a bunch of templates involved.

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[SteveRuble]

Oops, clicked the wrong button.

[msujew]

This looks really nice, thank you! Just a few minor improvements, see below.

[SteveRuble]

@msujew, sorry for the churn, but I realized there's a more intuitive behavior for the flags. Now --module means "create a Go module and put the generated package in it" and --package means "generate a package with the templated files at this location (which defaults to .)". So you can do --module example.com/foo/bar --package lang --language quux and get a new module with the generated code in the lang directory. If you don't provide --package it generates in the root of the module; if you don't provide --module it generates in the root of the current directory (assuming there's a go.mod in it). That seems better than the one-or-the-other pattern.

[msujew]

sorry for the churn

No worries - I also think that the latest iteration on package/module handling makes more sense!

[msujew]

That works great! Thanks a lot for the effort, much appreciated :)

[spoenemann]

We should put this behind a generate command arg.