Add support for perl language server (#5003)

* Fix perl test case clean-up
* Add support for perl language server

Co-authored-by: cos <cos>

ale_linters/perl/languageserver.vim

@@ -0,0 +1,43 @@
+" Authors: ophirr33 <coghlan.ty@gmail.com>, rymdbar <https://rymdbar.x20.se/>
+" Description: Perl::LanguageServer for perl, from cpan.org
+
+" This should have the same value as in perl.vim
+call ale#Set('perl_perl_executable', 'perl')
+" Please note that perl_perl_options does not exist here.
+
+function! ale_linters#perl#languageserver#GetProjectRoot(buffer) abort
+    " Makefile.PL, https://perldoc.perl.org/ExtUtils::MakeMaker
+    " Build.PL, https://metacpan.org/pod/Module::Build
+    " dist.ini, https://metacpan.org/pod/Dist::Zilla
+    let l:potential_roots = [ 'Makefile.PL', 'Build.PL', 'dist.ini', '.git' ]
+
+    for l:root in l:potential_roots
+        let l:project_root = ale#path#FindNearestFile(a:buffer, l:root)
+
+        if !empty(l:project_root)
+            return fnamemodify(l:project_root . '/', ':p:h:h')
+        endif
+    endfor
+
+    return fnamemodify(expand('#' . a:buffer . ':p:h'), ':p:h')
+endfunction
+
+call ale#Set('perl_languageserver_config', {})
+
+function! s:get_lsp_config(buffer) abort
+    " This tool doesn't kick in unless workspace/didChangeConfiguration is
+    " called, thus this function returning a fallback dict when there is no
+    " config.
+    let l:lsp_config = ale#Var(a:buffer, 'perl_languageserver_config')
+
+    return empty(l:lsp_config) ? { 'perl': { 'enable': 1 } } : l:lsp_config
+endfunction
+
+call ale#linter#Define('perl', {
+\   'name': 'languageserver',
+\   'lsp': 'stdio',
+\   'executable': {b -> ale#Var(b, 'perl_perl_executable')},
+\   'command': '%e -MPerl::LanguageServer -ePerl::LanguageServer::run',
+\   'lsp_config': {b -> s:get_lsp_config(b)},
+\   'project_root': function('ale_linters#perl#languageserver#GetProjectRoot'),
+\ })