feature(formatter): format ASTs with missing tokens or nodes

[MichaReiser]

Summary

Adds support for formatting ASTs with missing tokens or nodes which is the first step on our journey to support (partially) formatting source files with arbitrary syntax errors.

The main change is to return an Option<FormatElement> instead of a FormatElement from ToFormatElement.to_format_element if a node or token contains invalid content (e.g. a mandatory token or child node is missing). The parent has then the option to implement some recovery logic, for example inserting the node with its current formatting, or can propagate the formatting error (which most nodes will do).

This PR does not address the issue where tokens and/or nodes are present in places they aren't expected in a valid syntax tree. For example, the formatter messes up the following file

-----;
let x = 10;

becomes

let x = 10;

because script.items() only returns Stmts but the rslint parser doesn't generate a Stmt for ----. Supporting these cases will require significant changes to the rslint parser, which is why we address this issue separately. See my update comment on #1700 for a more extensive explanation of the problem and what my plans are.

Test Plan

Added a few spec tests but we should extend the test cases in the future.

[MichaReiser]

We need to bail out (return None) for tokens and children that are expected to be present in a valid sub-tree.

That means for mandatory fields (nodes and tokens) use formatter.format_token(&self.function_token()?)?); and only use

if let Some(token) = self.star_token() {
  tokens.push(formatter.format_token(&token)?)
}

for optional fields.

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[MichaReiser]

This will become formatter.format_token(&self.l_paren_token())? if we implement my suggestion that mandatory tokens and nodes should be inserted by the parser (and thus, we can change the field accessors for mandatory fields to non-optional types).

[ematipico]

Looks good to me!

[ematipico]

I find this comment hard to read, can it be reworded?