[Web] Add "threads"/"nothreads" feature tags to export presets

[Faless]

This allows selecting threads/nothreads gdextensions at export time (see godotengine/godot-cpp#1451).

As per my comment (godotengine/godot-cpp#1451 (comment)) it would be nice to provide also a nothreads feature flag to avoid relying on ordering in the gdextension file (and improve detection when using auto prefix).

See Faless@a17b075 for the relevant changes (happy to attach them to this PR).

[akien-mga]

As per my comment (godotengine/godot-cpp#1451 (comment)) it would be nice to provide also a nothreads feature flag to avoid relying on ordering in the gdextension file (and improve detection when using auto prefix).

See Faless@a17b075 for the relevant changes (happy to attach them to this PR).

I think that makes sense to add.

[Faless]

In common style we prefer positive names over negative names like nothreads.
Maybe singlethread? I am not picky.

nothreads was chosen for the binary name in #85939 , we should stay consistent with that if possible.

[mihe]

I went ahead and made a PR for a nothreads feature tag: #93563

[akien-mga]

Can be rebased on top of #93563 to add the Web exporter counterpart.

[Faless]

Can be rebased on top of #93563 to add the Web exporter counterpart.

Done

[akien-mga]

Thanks!