WAF: Extract definition of constants from Waf_Runner to Waf_Constants

[nateweller]

Changes proposed in this Pull Request:

• Moves define_mode() and define_share_data() from Waf_Runner to Waf_Constants.
• Renames Waf_Constants::define_constants() to Waf_Constants::define_bootstrap_constants(), and extracts the three constants that it defines into three distinct methods: define_waf_directory(), define_wpconfig_path(), and define_killswitch().

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
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Jetpack product discussion

peb6dq-mP-p2

Does this pull request change what data or activity we track or use?

No

Testing instructions:

• Proof read changes
• Ensure tests pass
• Check for regression in package functionality

[github-actions]

Are you an Automattician? You can now test your Pull Request on WordPress.com. On your sandbox, run bin/jetpack-downloader test jetpack update/waf-constants to get started. More details: p9dueE-5Nn-p2

[github-actions]

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[karovski]

Went through the changes, tests pass OK, and did a UI pass. LGTM!