Define types and functions in the proposal's template

Author: loganek

proposal-template.abi.md

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+# Types
+
+## <a href="#thread_id" name="thread_id"></a> `thread-id`: `u32`
+
+  Unique thread identifier.
+
+Size: 4, Alignment: 4
+
+## <a href="#start_arg" name="start_arg"></a> `start-arg`: `u32`
+
+  A reference to data passed to the start function (`wasi_thread_start()`) called by the newly spawned thread.
+
+Size: 4, Alignment: 4
+
+## <a href="#errno" name="errno"></a> `errno`: enum
+
+  Error codes returned by the `thread-spawn` function.
+
+Size: 1, Alignment: 1
+
+### Enum Cases
+
+- <a href="errno.eagain" name="errno.eagain"></a> [`eagain`](#errno.eagain)
+
+  TBD
+
+# Functions
+
+----
+
+#### <a href="#thread_spawn" name="thread_spawn"></a> `thread-spawn` 
+
+  Creates a new thread.
+##### Params
+
+- <a href="#thread_spawn.start_arg" name="thread_spawn.start_arg"></a> `start-arg`: [`start-arg`](#start_arg)
+##### Results
+
+- result<[`thread-id`](#thread_id), [`errno`](#errno)>
+

proposal-template.wit.md

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+# WASI threads API
+
+WASI threads is an API for thread creation.
+
+Its goal is to provide functions that allow implementation of a subset of `pthreads` API, but it doesn't aim to be 100% compatible with POSIX threads standard.
+
+
+## thread-id
+
+```wit
+/// Unique thread identifier.
+type thread-id = u32
+```
+
+## start-arg
+
+```wit
+/// A reference to data passed to the start function (`wasi_thread_start()`) called by the newly spawned thread.
+type start-arg = u32
+```
+
+## errno
+
+```wit
+/// Error codes returned by the `thread-spawn` function.
+enum errno {
+    /// TBD
+    eagain,
+}
+```
+
+## thread_spawn
+
+```wit
+/// Creates a new thread.
+thread-spawn: func(
+    /// A value being passed to a start function (`wasi_thread_start()`).
+    start-arg: start-arg,
+) -> result<thread-id, errno>
+```