[FEATURE] Custom conceal characters

[grepgrok]

Description of the Problem

There are some sequences where either there is no current concealment or I can think of a better but I can't just make it do that. For example, I would like to have \lVert a \rVert be concealed as || a || but it currently just shows the base LaTeX.

Description of the Solution

Add JSON like with the snippets mapping commands or strings of text to a concealment. For example,

{
    "\\lVert": " || ",
    "\\rVert": " || ",
}