Add with_wasm_pthread flag

build/rive_build_config.lua

diff --git a/build/rive_build_config.lua b/build/rive_build_config.lua
index 530c2d0..65ff9c8 100644
--- a/build/rive_build_config.lua
+++ b/build/rive_build_config.lua
@@ -119,6 +119,11 @@
 })
 
 newoption({
+    trigger = 'with_wasm_pthread',
+    description = 'Embed wasm directly into the js, instead of side-loading it.',
+})
+
+newoption({
     trigger = 'no-lto',
     description = 'Don\'t build with link time optimizations.',
 })
@@ -808,6 +813,12 @@
         linkoptions({ '-sSINGLE_FILE=1' })
     end
 
+    filter('options:with_wasm_pthread')
+    do
+        buildoptions({ '-pthread' })
+        linkoptions({ '-pthread' })
+    end
+
     filter('options:config=release')
     do
         buildoptions({ '-Oz' })