feat(linters): add shellcheck_ebuild

[ingenarel]

disclaimer: if this is not in the scope of this plugin, please tell me and i'll make a seperate plugin that adds this config for nvim-lint

shellcheck throws a lot of false SC2034 and SC2154 warnings for ebuilds, there has been an open issue for allowing pre declared external globals since 2015: httpcreate.shs://github.com/koalaman/shellcheck/issues/356 and a discussion about a fork which is also possibly now defunct: koalaman/shellcheck#2704

i have found a workaround for ebuilds and i was able to implement it's usage for neovim

in short: the small create.sh script generates a lua file that returns a string of all the ebuild and eclass vars exported, then the linter config creates a new .ebuild file in the cache with that's the original ebuild file, and the export data appended to them, because they're just exports shellcheck doesn't throw a warning on them, and on top of that it also stops false positives

i'll keep this as a draft for now because i still have a few other niche variables to add

[mfussenegger]

if this is not in the scope of this plugin, please tell me and i'll make a seperate plugin that adds this config for nvim-lint

This does indeed look a bit more than what's typically in scope for a linter definition. What would be okay is if you create a shellcheck_ebuild project that has it's own executable wrapping shellcheck with the logic here and then update this PR with a shellcheck_ebuild linter that calls that shellcheck_ebuild executable without any extra scripts needed in nvim-lint.