Allow loading preambles from file

[kimhanm]

Add an option to define the typst preambles in a file, similar to obsidian-latex-suite plugin.
See artisticat1/obsidian-latex-suite#39, and artisticat1/obsidian-latex-suite@3d8f31f.

[kimhanm]

There is a workaround to get typst to load a preamble from a file within the vault.
In the preamble blocks, put

#set text(fill: white, size: SIZE)
#set page(width: WIDTH, height: HEIGHT)

#import "./../../../../../../path/to/typst-preamble.md":*

the ../ are there to ensure that the preamble loads even if the file is not in the same directory as the preamble, and a few extra doesn't seem to impact it.

If you save the preamble as a .typfile, you won't get a bunch of #let tags in your database, but it will allow you to link macro definitions to your notes.