x/tools/gopls: modernizer for direct references to embedded fields in struct literals

[madelinekalil]

This is a go1.27 feature: #9859

Implementation guidance for the modernizer from @adonovan: #9859 (comment)

[gabyhelp]

Related Issues

• spec: direct reference to embedded fields in struct literals #9859

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[adonovan]

See also https://go.dev/cl/672355 for some code that may be helpful.