[release] v0.6.39

[yuanming-hu]

Related issue = #677

Full changelog:

• [refactor] Use PrimitiveType::type instead of DataType::type ([refactor] Use PrimitiveType::type instead of DataType::type #1926) (by Taichi Gardener)
• [example] Upgrade mass_spring_3d.py to Taichi THREE v0.0.5 ([example] Upgrade mass_spring_3d.py to Taichi THREE v0.0.5 #1919) (by 彭于斌)
• [async] [metal] Support async mode on Metal ([async] [metal] Support async mode on Metal #1920) (by Ye Kuang)
• [type] [refactor] Promote DataType to a class ([type] [refactor] Promote DataType to a class #1906) (by Yuanming Hu)
• [async] Partial SFG node GC (keep latest state writers/readers) ([async] Partial SFG node GC (keep latest state writers/readers) #1915) (by xumingkuan)
• [doc] Improve description of the pos parameter in gui ([Doc] Improve description of the pos parameter in gui #1904) (by qbao0x3273)

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