rework config system

[Techatrix]

Since configuration can be very sensible to changes it may be better to hold these changes back until after the 0.11.0 release.

These changes move some of the configuration setup into the initialize request which allows sending messages about bad config options that show up in the user interface (in VS-Code at least).

fixes #1251
fixes #1238
fixes #1129
fixes #994
fixes #1104

[leecannon]

Using "zig" as the zig path in order to use the zig in the PATH causes this check to fail, even though that works perfectly fine.

Other than that minor annoyance this PR looks good.

[llogick]

It would seem that the .{} param here invalidates some of the zls.json settings?
Causes try server.resolveConfiguration(config_arena, &new_cfg); to reevaluate the corresponding ?const []u8s as if they haven't been set.

[Trace - 2:49:05 PM] Sending request 'initialize - (0)'.
info : ( main ): Enabled debug logging.
info : ( main ): Starting ZLS 0.12.0-dev.17+4c3236b @ '/home/rad/lab/zls/npdm/zig-out/bin/zls'
info : (server): set config option 'semantic_tokens' to 'Config.Config__enum_5911.none' // Custom
info : (server): set config option 'warn_style' to 'true' // Custom
info : (server): set config option 'highlight_global_var_declarations' to 'true' // Custom
info : (server): set config option 'skip_std_references' to 'true' // Custom
info : (server): set config option 'prefer_ast_check_as_child_process' to 'false' // Custom
info : (server): set config option 'builtin_path' to '/home/rad/.cache/zls/builtin.zig' // Auto
info : (server): set config option 'zig_lib_path' to '/home/rad/lab/zig/build/stage3/lib/zig' // Auto
info : (server): set config option 'zig_exe_path' to '/home/rad/lab/zig/build/stage3/bin/zig' // Custom
info : (server): set config option 'build_runner_path' to '/home/rad/.cache/zls/build_runner.zig' // Auto
info : (server): set config option 'global_cache_path' to '/home/rad/.cache/zls' // Auto
info : (server): set config option 'build_runner_global_cache_path' to '/home/rad/.cache/zig' // Auto
info : (server): client is 'VSCodium-1.81.0'
info : (server): Server.ClientCapabilities{ .supports_snippets = true, .supports_apply_edits = true, .supports_will_save = true, .supports_will_save_wait_until = true, .supports_publish_diagnostics = true, .supports_code_action_fixall = false, .hover_supports_md = true, .completion_doc_supports_md = true, .label_details_support = true, .supports_configuration = true, .supports_workspace_did_change_configuration_dynamic_registration = true, .supports_textDocument_definition_linkSupport = true }
info : (server): offset encoding: utf-16
info : (server): set config option 'zig_lib_path' to '/home/rad/lab/zig/lib' // Auto
info : (server): set config option 'zig_exe_path' to '/home/rad/lab/zig/build/stage4/bin/zig' // Auto from PATH

[Techatrix]

does #1404 resolve your issue?