docs: Fungible Token for AI conversation

[petersalomonsen]

Updating the docs in examples/fungibletoken/README.md with a step by step instruction for the flow of starting an AI conversation, tracking token usage, and refunding unspent tokens.

Other updates in this PR:

• Add a view method view_js_func for calling read-only javascript functions
• Add view methods for tracking AI conversation balance in the example

See a brief demo video here: https://youtu.be/-R_zok--L_A?si=AkBl4IBjk6dbXDiw

The screenshot below is from the ongoing Pull Request in petersalomonsen/near-openai#11

[petersalomonsen]

@race-of-sloths include

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Reviewer	Score
@Tguntenaar	3

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[Tguntenaar]

LGTM

[Tguntenaar]

@race-of-sloths score 3