add options to show fenced block source

[trebor]

adds global and front-matter options to show fenced block source code. i am happy to change up the syntax, and/or semantics if folks have good alternatives, but here is what i have implemented. btw: i considered a boolean, but went with the "show" string to indicate the specific action. in observablehq.config.ts add:

  ...
  blocks: "show",
  ...

and in front-matter:

---
blocks: show
---

in both cases, values other than "show" will have no effect.

echo takes precedence over front-matter blocks, which takes precedence over observablehq.config.ts blocks.

hmmm...

as i look back at the issues, it may be this option was always intended to be called 'echo' in config and front-matter. which would make sense. let me know if i should switch over to that.

closes #156

[mbostock]

Yes, it should be echo: false, but there are a couple other considerations, namely

1. whether the echo option should be scoped under a execute object like Quarto execution options (though I don’t think I’d use the name execute) so we can have other options there too, namely run: false if you want to disable run by default and

2. language-specific options (e.g., turning on echo for js, but turning it off for html). So maybe the scope should be js or html for language-specific options, and top-level for all languages, with suitable precedence.

[trebor]

i've not been able to come up with any great alternatives to an "execute". maybe 'construct'?

for echo the spec could take a boolean, or a list of language tags like:

echo: [js, html]

[mbostock]

Sorry for not being clear. The alternative I was suggesting was to use the language. So,

js:
  echo: true

means turn on echo true by default for js blocks. Whereas

echo: true

means turn it on for all blocks.