go/token: replace 'go' keyword with 'async' for improved readability

[ppmpreetham]

Replaced the 'go' keyword with 'async' in the token definitions due to readability

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This change modifies Go to use 'async' in place of 'go' in the token
definitions, aiming to improve readability and align the language
closer with conventions from other asynchronous programming languages.

If merged, this will update all related token definitions and parsing
logic to support 'async' as the new keyword for launching concurrent
processes. Tests have been updated to reflect the change.

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