[JS] Dotprompt always looks in prompts/ even when initialized with other directory

[kevinthecheung]

Describe the bug

Dotprompt always looks in prompts/ even when initialized with other directory.

(the library loads all of the prompts in the configured directory when you initialize it but thereafter if you reference a prompt that's not found it looks in the default location rather than the one you specified at initialization)

To Reproduce

1. Put a hello.prompt file in prompts/
2. Initialize plugin, specifying a different dir: dotprompt({ dir: './prompts2' })
3. Try:

   const helloPrompt = await prompt('hello');
   const response = await helloPrompt.generate({}); 
   

   It still succeeds.
4. Try with promptRef('hello'). It also succeeds.
5. OTOH the devUI doesn't load the prompt

Expected behavior

3 and 4 should be errors. 5 is expected.

Screenshots
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Runtime (please complete the following information):

• OS: [e.g. Linux, MacOS]
• Version [e.g. 22]

** Node version

• run node --version at paste here

Additional context
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