feat(shlvl): Add shlvl module (#1385)

* initial commit of support for shlvl

* documentation for shlvl

* use a symbol instead

* test coverage for shlvl

* actually disable when the config says to

* fix docs

* tweak defaults some

* refactor from pr comments

* redisable

* return early to avoid indenting

* make default suffix empty space

* fixing tests after suffix change

* updating docs for format

* making shlvl compatible with formatting

* adding variables table for shlvl

* removing extra line

* doc clarity

src/configs/mod.rs

@@ -39,6 +39,7 @@ pub mod purescript;
 pub mod python;
 pub mod ruby;
 pub mod rust;
+pub mod shlvl;
 pub mod singularity;
 mod starship_root;
 pub mod swift;