feat: add Syntax::layers_for_byte_range

[nik-rev]

When I tried to migrate the comment continuation PR (helix-editor/helix#12759) to use the new tree-house bindings, I encountered a problem:

When continuing comments, we look at the smallest injection layer that includes the selection. This layer is the comment language itself, which has no tokens. We need to ignore it and try the next layer. But there's no way to do that currently

I think returning a list of layers from largest to smallest that include the byte range is a good API for tree-house

I updated my helix PR to make use of this PR: https://github.com/helix-editor/helix/blob/f07e6973fedb2fed4ff7f0e1a9d166bea7bedcab/helix-core/src/comment.rs#L58-L77

Public API

impl Syntax {
    /// # Returns
    ///
    /// A list of layers which **fully include** the byte range `start..=end`,
    /// in decreasing order based on the size of each layer.
    ///
    /// The first layer is the `root` layer.
    pub fn layers_for_byte_range(&self, start: u32, end: u32) -> Iterator<Item = Layer>;
}

[the-mikedavis]

We should avoid the Vec if we can and have this be an iterator instead.

Alternatively we could expose parent on LayerData. You could use that with the existing layer_for_byte_range to find all layers.

[nik-rev]

We should avoid the Vec if we can and have this be an iterator instead.

Alternatively we could expose parent on LayerData. You could use that with the existing layer_for_byte_range to find all layers.

I assume you'd like to avoid the Vec to not have to make extra allocations?
In any case I updated the PR to use iter::from_fn so it returns Iterator<Item = Layer> now

[nik-rev]

Also I'm curious why did you choose to receive start: u32, end: u32 rather than range: impl RangeBounds<u32>?