Tree-sitter grammar: support if/else expressions

[OceanOak]

Changelog:

Tree-sitter-darklang
- Update the grammar to support if/else statements
- Add an external scanner to handle indentation

#5321

[StachuDotNet]

I wonder if we could write a function in our grammar.js that would abstract over this pattern

choice(
  seq(
    $.indent,
    repeat1(field("then_expression", $.expression)),
    $.dedent,
  ),
  field("then_expression", $.paren_expression),
),

like

let maybeWithIndentDedent (thing) = 
  choice(
    seq($.indent, thing, $.dedent),
    thing,
  ),

[OceanOak]

I wonder if we could write a function in our grammar.js that would abstract over this pattern

choice(
  seq(
    $.indent,
    repeat1(field("then_expression", $.expression)),
    $.dedent,
  ),
  field("then_expression", $.paren_expression),
),

like

let maybeWithIndentDedent (thing) = 
  choice(
    seq($.indent, thing, $.dedent),
    thing,
  ),

Some initial thoughts:

• The function may not recognize $.indent and $.dedent since they're defined after the function's declaration
  i.e.

let maybeWithIndentDedent ....

module.exports = grammar({
  name: "darklang",
  
  // I believe we can't declare that function here
  ...

so, the function would probably be more like

let maybeWithIndentDedent = (indent, thing, dedent) =>
  choice(seq(indent, thing, dedent), thing);

• Losing the field names would make it challenging to capture in the parser, no?

Unrelated note:
we still need to wrap the 'then' expression in parentheses (only when writing an inline if/else expression) due to conflicts. planning to address this when fixing apply...

[StachuDotNet]

it's neat that you needed a paren_expression here, but not in the equivalent else_expression below. This tree-sitter conflict stuff continues to be a mystery to me.

[StachuDotNet]

Since this PR is just focused on the parser, this is good -- could we follow this PR up with one focused on the pretty-printing of EIfs?
such that:
if an if has an else, whose root expression is an if as well, we format like:

if cond then
  expr
else if cond2 then
  expr2

?

[StachuDotNet]

(it might turn out that that code might need to 'track' indentation to do its job correctly)

[OceanOak]

Yes, I tried to make it work within this PR, but encountered some issues with the indentations