Add web export preset

This preset is just the default settings for the Web target (as of
v4.3.stable.flathub [77dcf97d8], at least).

The important line is:

variant/thread_support=false

This selects a build of the Godot web runtime that does not use threads; and
hence does not use SharedArrayBuffer; and hence can be used on GitHub Pages
where the necessary headers to allow SharedArrayBuffer to be used cannot be
set.

https://phabricator.endlessm.com/T35699

Author: wjt

export_presets.cfg

@@ -0,0 +1,41 @@
+[preset.0]
+
+name="Web"
+platform="Web"
+runnable=false
+advanced_options=false
+dedicated_server=false
+custom_features=""
+export_filter="all_resources"
+include_filter=""
+exclude_filter=""
+export_path=""
+encryption_include_filters=""
+encryption_exclude_filters=""
+encrypt_pck=false
+encrypt_directory=false
+script_export_mode=2
+
+[preset.0.options]
+
+custom_template/debug=""
+custom_template/release=""
+variant/extensions_support=false
+variant/thread_support=false
+vram_texture_compression/for_desktop=true
+vram_texture_compression/for_mobile=false
+html/export_icon=true
+html/custom_html_shell=""
+html/head_include=""
+html/canvas_resize_policy=2
+html/focus_canvas_on_start=true
+html/experimental_virtual_keyboard=false
+progressive_web_app/enabled=false
+progressive_web_app/ensure_cross_origin_isolation_headers=true
+progressive_web_app/offline_page=""
+progressive_web_app/display=1
+progressive_web_app/orientation=0
+progressive_web_app/icon_144x144=""
+progressive_web_app/icon_180x180=""
+progressive_web_app/icon_512x512=""
+progressive_web_app/background_color=Color(0, 0, 0, 1)