docs: add more details on use of .d.tl files (cont.)

[pdesaulniers]

This PR does a few things:

• Merge https://github.com/teal-language/tl/pull/143
• Apply suggested tweaks to the PR
• Add a concrete example which shows how to use global_env_dev
• Put this new information regarding declaration files in a separate document (declaration_files.md)
• Add a link to this document in the README
• Add a mention about tlconfig.lua and the compiler options page in the README (since include_dir and global_env_def can be good to know when working with declaration files)

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