[NTG-context] OpenType contextual alternates disabled by default

[Max Chernoff via ntg-context <ntg-context@ntg.nl>]

Hi Hans,

OpenType contextual alternates ("calt") seem to be disabled by default
in ConTeXt. However, the OpenType spec recommends that this is enabled
by default

    https://learn.microsoft.com/en-us/typography/opentype/spec/features_ae#tag-calt

and it's enabled by default in both HTML and HarfBuzz

    https://drafts.csswg.org/css-fonts/#default-features

    https://harfbuzz.github.io/shaping-opentype-features.html

Changing this default in ConTeXt probably isn't possible due to
backwards compatibility concerns, so this is more of an FYI than a
request for any changes to be made.

Background: this came up in a recent TeX.SE question

    https://tex.stackexchange.com/a/750767

where a new user was surprised by ConTeXt's default setting since it
makes some fonts render differently than other programs.

Thanks,
-- Max
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