fix: Modifies read to handle large LS messages

[cskeeters]

read is written correctly to handle the case where multiple JSON messages are returned from the Language Server (LS) with new lines as the messages separator. However, as the Typst source gets large messages from the LS, the messages become too large to fit in a single chunk of data read from stdout of the LS. LibUV makes no guarentee that the message will end on a newline, so the left over data needs to be saved. In this case, vim logs an error because data:find returns nil into s and the code tries to add 1 to a nil value.

I got this to work by having a read_buffer outside the function that can persist the unused portion of the previous read chunk and apply it to the next message before parsing.

To reproduce this error, create test.typ with:

= Introduction 0

Then make 99 copies of this line. I incremented the number each time.

qq<C-a>yypq
99@q

Run TypstPreview, then try to insert text. You'll get one or two related errors in vim

Error executing vim.schedule lua callback: ...typst-preview.nvim/lua/typst-preview/servers/factory.lua:
180: attempt to perform arithmetic on local 's' (a nil value)
stack traceback:
        ...typst-preview.nvim/lua/typst-preview/servers/factory.lua:180: in function ''
        vim/_editor.lua: in function ''
        vim/_editor.lua: in function <vim/_editor.lua:0>

This cutoff may vary by OS. I'm on Mac OS Sonoma 14.5.

[cskeeters]

This print statement should be removed from the final version.

[chomosuke]

Lgtm. Thank you for your work!!