Add a markdown pre-processor for templating prompts at runtime

[extremeheat]

Add a templating system, which is described in doc (and now used internally for Google AI Studio Playground chat handling).

To insert variables, you use %%%(VARIABLE_NAME)%%%. To insert a string based on a specific condition, you use %%%[...] if CONDITION%%% or with an else clause, %%%[...] if BOOLEAN_VAR else [...]%%%. Note that the square brackets work like quotation mark strings in programming languages, so you can escape them with a backslash, like \].

There are also if statements, which can be used to conditionally include or exclude parts of the markdown.

For example, the line:

Your name is %%%(NAME)%%%, and you answer questions for the user%%%[, based on your prompt] if HAS_PROMPT%%%.
You are running over %%%[the Google AI Studio playground] if IS_AI_STUDIO else [the %%%(LLM_NAME)%%% API]%%%.

Would be loaded in JS like this:

const { importPromptSync } = require('langxlang')
const prompt = importPromptSync('path-to-prompt.md', { NAME: 'Omega', HAS_PROMPT: true, IS_AI_STUDIO: false, LLM_NAME: 'Gemini 1.5 Pro' })

And would result in:

Your name is Omega, and you answer questions for the user, based on your prompt.
You are running over the Gemini 1.5 Pro API.

[extremeheat]

/makerelease