Allow prompt templates to be overridden in the zed configuration directory (#15887)

I need this to refine our prompts on the fly as I work.

Release Notes:

- Templates for prompts driving inline transformation in editors and the
terminal can now be overridden in the `~/.config/zed/prompts/templates`
directory. This is an advanced feature, and prevents you from getting
upstream changes. It's intended for use by Zed developers.

assets/prompts/content_prompt.hbs

@@ -0,0 +1,61 @@
+{{#if language_name}}
+Here's a file of {{language_name}} that I'm going to ask you to make an edit to.
+{{else}}
+Here's a file of text that I'm going to ask you to make an edit to.
+{{/if}}
+
+{{#if is_insert}}
+The point you'll need to insert at is marked with <insert_here></insert_here>.
+{{else}}
+The section you'll need to rewrite is marked with <rewrite_this></rewrite_this> tags.
+{{/if}}
+
+<document>
+{{{document_content}}}
+</document>
+
+{{#if is_truncated}}
+The context around the relevant section has been truncated (possibly in the middle of a line) for brevity.
+{{/if}}
+
+{{#if is_insert}}
+You can't replace {{content_type}}, your answer will be inserted in place of the `<insert_here></insert_here>` tags. Don't include the insert_here tags in your output.
+
+Generate {{content_type}} based on the following prompt:
+
+<prompt>
+{{{user_prompt}}}
+</prompt>
+
+Match the indentation in the original file in the inserted {{content_type}}, don't include any indentation on blank lines.
+
+Immediately start with the following format with no remarks:
+
+```
+{{INSERTED_CODE}}
+```
+{{else}}
+Edit the section of {{content_type}} in <rewrite_this></rewrite_this> tags based on the following prompt:
+
+<prompt>
+{{{user_prompt}}}
+</prompt>
+
+{{#if rewrite_section}}
+And here's the section to rewrite based on that prompt again for reference:
+
+<rewrite_this>
+{{{rewrite_section}}}
+</rewrite_this>
+{{/if}}
+
+Only make changes that are necessary to fulfill the prompt, leave everything else as-is. All surrounding {{content_type}} will be preserved.
+
+Start at the indentation level in the original file in the rewritten {{content_type}}. Don't stop until you've rewritten the entire section, even if you have no more changes to make, always write out the whole section with no unnecessary elisions.
+
+Immediately start with the following format with no remarks:
+
+```
+{{REWRITTEN_CODE}}
+```
+{{/if}}

assets/prompts/terminal_assistant_prompt.hbs

@@ -0,0 +1,18 @@
+You are an expert terminal user.
+You will be given a description of a command and you need to respond with a command that matches the description.
+Do not include markdown blocks or any other text formatting in your response, always respond with a single command that can be executed in the given shell.
+Current OS name is '{{os}}', architecture is '{{arch}}'.
+{{#if shell}}
+Current shell is '{{shell}}'.
+{{/if}}
+{{#if working_directory}}
+Current working directory is '{{working_directory}}'.
+{{/if}}
+{{#if latest_output}}
+Latest non-empty terminal output:
+{{#each latest_output as |line|}}
+{{line}}
+{{/each}}
+{{/if}}
+Here is the description of the command:
+{{{user_prompt}}}