fix: prevent reach-router dep requirement for v2

[taty2010]

Summary

When using the current plugin version with Gatsby V2, users were running into this error:

With the previous Dynamic import update, there were issues that could have emerged when placing the deleteFunctions within the conditional. This new change converts the getApiHandler function (where reach-router is being called) into a string and prevents reach-router from being required as a dependency for v2 sites.

makeApiHandler contains getApiHandler and is only called when creating a function.

Test plan

1. Can test it with a V2 site and make sure the above error doesn't appear
   • Example site: https://github.com/taty2010/gatsby-v2-test (will need to uninstall @gatsbyjs/reach-router)

Relevant links (GitHub issues, Notion docs, etc.) or a picture of cute animal

Ref: #556

Standard checks:

• Check the Deploy Preview's Demo site for your PR's functionality
• Add docs when necessary

---

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[nickytonline]

This looks good to me since we tested it together. 😎 I'll let someone else give some 👀 for approval as well though.

[ascorbic]

Neat solution!

[ascorbic]

Could you give this PR a title that obeys the conventional commits format