Add terraform-lsp integration (#2758)

* Add terraform-lsp integration

https://github.com/juliosueiras/terraform-lsp

* Add tests & docs for terraform-lsp integration

terraform_langserver_options setting added to send custom flags to
terraform-lsp.

Vader tests have been added to test custom executable, custom flags, and
finding the project root. All tests pass.

Initial documentation has been added for the above.

Resolves dense-analysis/ale#2758, juliosueiras#57

* Fix tag alignment

Co-authored-by: = <Aubrey.S.Lavigne@gmail.com>
Co-authored-by: w0rp <w0rp@users.noreply.github.com>

ale_linters/terraform/terraform_lsp.vim

@@ -0,0 +1,25 @@
+" Author: OJFord <dev@ojford.com>
+" Description: terraform-lsp integration for ALE (cf. https://github.com/juliosueiras/terraform-lsp)
+
+call ale#Set('terraform_langserver_executable', 'terraform-lsp')
+call ale#Set('terraform_langserver_options', '')
+
+function! ale_linters#terraform#terraform_lsp#GetCommand(buffer) abort
+    return '%e'
+    \   . ale#Pad(ale#Var(a:buffer, 'terraform_langserver_options'))
+endfunction
+
+function! ale_linters#terraform#terraform_lsp#GetProjectRoot(buffer) abort
+    let l:tf_dir = ale#path#FindNearestDirectory(a:buffer, '.terraform')
+
+    return !empty(l:tf_dir) ? fnamemodify(l:tf_dir, ':h:h') : ''
+endfunction
+
+call ale#linter#Define('terraform', {
+\   'name': 'terraform_lsp',
+\   'lsp': 'stdio',
+\   'executable': {b -> ale#Var(b, 'terraform_langserver_executable')},
+\   'command': function('ale_linters#terraform#terraform_lsp#GetCommand'),
+\   'project_root': function('ale_linters#terraform#terraform_lsp#GetProjectRoot'),
+\   'language': 'terraform',
+\})