enable setting of template placeholder for text format based on file extension

[castedo]

This is additional functionality beyond #2. Based on the file extension of the source file passed to copyaid, enable the setting of a template placeholder for the chat_system message setting.

Maybe the mapping between filename extension and text to be inserted should be direct like:

".md" -> "Markdown and HTML"
".tex" -> "LaTeX"

Note how "Markdown and HTML" could perform better in the prompt for GPT even though to a human, conceptually, "Markdown" should suffice, in theory.

Or maybe there should be a MIME type indirection in between:

".md" -> "text/markdown" -> "Markdown and HTML"
".tex" -> "application/x-latex" -> "LaTeX"

These mapping could be exclusively in the user copyaid.toml config file or the prompt query settings file. My inclination is that the mapping

".md" -> "text/markdown"
".tex" -> "application/x-latex"

should be in the user config copyaid.toml and the mapping

"text/markdown" -> "Markdown and HTML"
"application/x-latex" -> "LaTeX"

should be in the settings file, along with the OpenAI API settings like the prompt template and the GPT model setting.

[castedo]

Idea for new settings of user config file (`copyaid.toml')

[copybreak]
ids = ["copybreak", "cbr"]

[formats]
default = "markdown"

[formats.markdown]
mime_type = "text/markdown"
extensions = ["md"]
copybreak.delim = ["<!--", "-->"]

[formats.tex]
mime_type = "application/x-latex"
extensions = ["tex"]
copybreak.delim = "%%"

[castedo]

Rather than placing mime_type directly under the format, consider enable generic setting of instruction parameters per format via something like:

[formats.markdown]
instruction.parameters = { mime_type = "text/markdown" }