fix(xgotext): extract static string identifiers safely

[leonelquinteros]

Summary

Supersedes #128 and incorporates the original contribution from @githubzhaoqian, plus follow-up fixes, tests, and documentation.

• resolve string constants, aliases, and declaration-time variable values
• preserve identifier-to-value mapping for multi-name declarations
• skip values that are dynamic or reassigned before use
• safely reject unsupported getter arguments without panicking
• cover directory and package-tree extraction paths

Validation

ok github.com/leonelquinteros/gotext (cached)
? github.com/leonelquinteros/gotext/cli/xgotext [no test files]
? github.com/leonelquinteros/gotext/cli/xgotext/fixtures [no test files]
? github.com/leonelquinteros/gotext/cli/xgotext/fixtures/pkg [no test files]
ok github.com/leonelquinteros/gotext/cli/xgotext/parser (cached)
ok github.com/leonelquinteros/gotext/cli/xgotext/parser/dir (cached)
ok github.com/leonelquinteros/gotext/cli/xgotext/parser/pkg-tree (cached)
ok github.com/leonelquinteros/gotext/plurals (cached)

Closes #128