fix(zls): inclide build.zig in root dir search

File `build.zig` by convention sits at the root dir and thus can be used
to correctly identify the root directory.

This is particularly useful if `zls` has `enable_build_on_save` set to
`true`, since then the language server uses workspace path (root_dir) to
execute `zig build` command, output of which is then used to publish
diagnostic errors.

Without `build.zig` root directory can incorrectly be deduced some
layers too high at the root of the git repo. In such case
`enable_build_on_save` silently fails and does not report build error
diagnostics.

doc/server_configurations.md

@@ -12048,7 +12048,7 @@ require'lspconfig'.zls.setup{}
   ```
   - `root_dir` : 
   ```lua
-  util.root_pattern("zls.json", ".git")
+  util.root_pattern("zls.json", "build.zig", ".git")
   ```
   - `single_file_support` : 
   ```lua

doc/server_configurations.txt

@@ -12048,7 +12048,7 @@ require'lspconfig'.zls.setup{}
   ```
   - `root_dir` : 
   ```lua
-  util.root_pattern("zls.json", ".git")
+  util.root_pattern("zls.json", "build.zig", ".git")
   ```
   - `single_file_support` : 
   ```lua

lua/lspconfig/server_configurations/zls.lua

@@ -4,7 +4,7 @@ return {
   default_config = {
     cmd = { 'zls' },
     filetypes = { 'zig', 'zir' },
-    root_dir = util.root_pattern('zls.json', '.git'),
+    root_dir = util.root_pattern('zls.json', 'build.zig', '.git'),
     single_file_support = true,
   },
   docs = {
@@ -14,7 +14,7 @@ https://github.com/zigtools/zls
 Zig LSP implementation + Zig Language Server
         ]],
     default_config = {
-      root_dir = [[util.root_pattern("zls.json", ".git")]],
+      root_dir = [[util.root_pattern("zls.json", "build.zig", ".git")]],
     },
   },
 }