docs(spec): add planning and language specification

[Matographo]

Summary

• add product requirements for the current Prebyte direction
• add a language specification draft for template syntax and semantics
• add an issue backlog document to support milestone and issue planning

Notes

• this PR adds planning and specification documents only
• it does not change the current implementation or existing README on main
• the spec includes safe missing-value propagation for chained access such as test[0].hello.name

Included Files

• REQUIREMENTS.md
• LANGUAGE_SPEC.md
• ISSUE_BACKLOG.md