docs: use The XGo Authors (xgo.dev) as new copyright info

[xgopilot]

Requested by @luoliwoshang

Closes #627

Summary

• Updated copyright information from "The GoPlus Authors (goplus.org)" to "The XGo Authors (xgo.dev)" across 14 source files

Files Changed

• _xtool/internal/libclang/basic.go
• _xtool/internal/libclang/clang.go
• _xtool/llcppsigfetch/llcppsigfetch.go
• _xtool/llcppsymg/llcppsymg.go
• ast/ast.go
• cl/blockctx.go
• cl/compile.go
• cl/decl.go
• cl/type_and_var.go
• cmd/gogensig/gogensig.go
• cmd/llcppg/llcppg.go
• parser/iparser.go
• parser/parser.go
• tool/gengo.go

[luoliwoshang]

LGTM

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.74%. Comparing base (9b01925) to head (09ffefd).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #628   +/-   ##
=======================================
  Coverage   84.74%   84.74%           
=======================================
  Files          27       27           
  Lines        2314     2314           
=======================================
  Hits         1961     1961           
  Misses        310      310           
  Partials       43       43           

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