Ligatures should be disabled when letter spacing is enabled

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As the title suggests, ligatures should be disabled when letter spacing is requested. Otherwise, the clusters that are sent to breaking represent ligatures that cannot be broken up.

For example, in the below test, we produce this image. Note how "ffi" in office are out of place and how the "fi" and "fl" ligatures are not broken up like "ff".

```rs
#[test]
fn interaction_letter_spacing_ligatures() {
let mut env = TestEnv::new(test_name!(), None);
let text = samples::LIGATURES;

// Without letter spacing - ligatures should form (if font supports them)
let features_on = FontFeatures::List(Cow::Borrowed(&[FontFeature {
tag: Tag::new(b"liga"),
value: 1,
}]));

let mut builder_no_spacing = env.ranged_builder(text);
builder_no_spacing.push_default(StyleProperty::FontFeatures(features_on.clone()));
builder_no_spacing.push_default(StyleProperty::LetterSpacing(0.0));
let mut layout_no_spacing = builder_no_spacing.build(text);
layout_no_spacing.break_all_lines(None);
layout_no_spacing.align(None, Alignment::Start, AlignmentOptions::default());

env.with_name("no_spacing")
.check_layout_snapshot(&layout_no_spacing);

// With letter spacing - ligatures SHOULD break
let mut builder_with_spacing = env.ranged_builder(text);
builder_with_spacing.push_default(StyleProperty::FontFeatures(features_on));
builder_with_spacing.push_default(StyleProperty::LetterSpacing(2.0));
let mut layout_with_spacing = builder_with_spacing.build(text);
layout_with_spacing.break_all_lines(None);
layout_with_spacing.align(None, Alignment::Start, AlignmentOptions::default());

env.with_name("with_spacing")
.check_layout_snapshot(&layout_with_spacing);
}
```

Seen in https://github.com/linebender/parley/pull/503

Start by running the interaction_letter_spacing_ligatures test shown in the issue and compare its no_spacing and with_spacing snapshots. Trace the layout path used by that test; done means ligatures form without letter spacing but are disabled and breakable when letter spacing is nonzero.

Written by the indexing model from the issue text.