Recognise async def method headers in worder

[gadievron]

Description

worder's is_a_class_or_function_name_in_header only recognised the def and
class header prefixes, so an async def method header was not treated as a
definition. As a result find_definition for an async def method called via
self resolved to the call site instead of the def, and Rename from the
call did not reach the def.

class C:
    async def target(self):
        pass
    async def run(self):
        await self.target()   # find_definition here -> the call site, not `def target`

Module-level async def already resolved correctly (different path); only method
resolution via self was affected.

Fix

Add "async def" to the recognised headers and normalise internal whitespace so
async def / async\tdef / line-continued forms also match:

return " ".join(prev_word.split()) in ["def", "class", "async def"]

The predicate reads the blanked real_code, so string/comment text can't spoof
it, and only those three exact strings match — no false-positive surface. The fix
is version-independent (async def has been valid since Python 3.5).

Tests cover find_definition on an async method (canonical and tab/extra-space
spellings), an end-to-end Rename, and two regression controls (sync methods and
module-level async defs still resolve).

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Checklist

• I have added tests that prove my fix is effective
• I have updated CHANGELOG.md