Add firstLines handlebars helper

[kym6464]

This PR adds a custom Handlebars helper called firstLines which works like the unix head command. This is for directories that contain .csv or .jsonl files since you only need to pass the first few lines to the LLM to have it understand the shape of the data, and the rest fills up the context for no reason.

This is best explained by example:

Example 1: Base Case

This shows the folder structure that I'm using for testing with a basic handlebars template.

template.hbs

Project Path: {{ absolute_code_path }}

Source Tree: 

{{ source_tree }}

{{#each files}}
{{#if code}}
{{path}}:

{{code}}

{{/if}}
{{/each}}

output

Project Path: test_dir

Source Tree:

test_dir
├── code.js
└── data.csv

test_dir/code.js:

function main() {
	console.log('hello world');
}

main();

test_dir/data.csv:

id,color
1,red
2,green
3,blue
1,red
2,green
3,blue
1,red
2,green
3,blue

Example 2: using firstLines

This example uses the firstLines helper by modifying the template in the following way:

- {{code}}
+ {{firstLines code 5}}

template_custom.hbs

Project Path: {{ absolute_code_path }}

Source Tree: 

{{ source_tree }}

{{#each files}}
{{#if code}}
{{path}}:

{{firstLines code 5}}  

{{/if}}
{{/each}}

output

Project Path: test_dir

Source Tree: 

test_dir
├── code.js
└── data.csv

test_dir/code.js:

```js
function main() {
	console.log('hello world');
}

test_dir/data.csv:

id,color
1,red
2,green
3,blue

Next Steps / Questions

I would like to figure out a way to selectively apply the firstLines helper to data files such as .csv and .jsonl. And ideally you could selectively exclude certain files where you do in fact want the entire thing in the context. Any thoughts on how to approach this?

I think it would make sense to update website/src/content/docs/docs/tutorials/learn_templates.mdx as part of this PR, does that sound ok? I was thinking of adding a "data analysis" geared template example to demonstrate how this is useful.

Disclaimer: this is my first experience with "writing" Rust (I had Claude write the code and verified functionality) so please let me know if there are any issues.

[kym6464]

@ODAncona for review