[fea-rs] Support inline alternate substitution in contextual rules

[anthrotype]

fea-rs currently rejects inline alternate substitution rules in contextual (chained) context:

  feature calt {
      sub a' comma from [A a.alt1 a.alt2];
      sub b' comma from [B b.alt];
  } calt;

  error: alternate substition rules cannot be specified inline
    |
  6 |     sub a' comma from [A a.alt1 a.alt2];
    |                  ^^^^

Although exotic, this is valid OpenType Feature File syntax. fonttools has supported compiling this since 2016 (fonttools/fonttools@db49f20d6, PR fonttools/fonttools#445). Just today, I added an optimization to reuse the same nested AlternateSubst lookup when consecutive chained alternate rules share the same context (fonttools/fonttools@0903764f8, PR fonttools/fonttools#4061).

In order to match feaLib, fea-rs should:

1. Parse and compile sub <glyph>' <context> from <alternates>; by creating a nested AlternateSubst lookup
2. Reuse the same nested lookup when consecutive rules share the same context

fonttools has a test for this: fontTools/Tests/feaLib/data/contextual_merge_alternate.{fea,ttx}.

Discovered while working on #1922 (contextual rule merging).