Escape double curly braces in prompt template strings

[timrwilliams]

Resolves #42

---

Simple approach to not include input variables denoted by double curly braces. As per the f-string syntax double curly braces are replaces by single curly braces in the output.

prompt = Prompt::PromptTemplate.new(template: "Tell me a {adjective} joke. Return in JSON in the format {{joke: 'The joke'}}", input_variables: ["adjective"])
prompt.format
=> "Tell me a {adjective} joke. Return in JSON in the format {joke: 'The joke'}"
irb(main):006:0>

Tested via new spec and also ran against all current Prompt examples on langchain-hub to ensure compatibility with wider ecosystem.

[andreibondarev]

Nitpicking: variable.start_with?("{") to improve the readability perhaps.

[andreibondarev]

@timrwilliams Just 1 comment.

Could you please upmerge your branch and also fix the failing specs? I think it's just the standardrb check failing.

Also -- would you please add some code comments referencing the intention behind this fix? Thank you!

[andreibondarev]

@timrwilliams Looks great, thank you!!

[andreibondarev]

@timrwilliams Looks great, thank you!!