lsp/source: enable experimentalPostfixCompletions by default

Leave the options flag so people can disable it for now if needed.

Updates golang/go#39507.

Change-Id: I78bbac157caa18c5d9a8e2ffe1a5c5eba4c6c30f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/317389
Run-TryBot: Muir Manders <muir@mnd.rs>
Run-TryBot: Rebecca Stambler <rstambler@golang.org>
gopls-CI: kokoro <noreply+kokoro@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>
Trust: Rebecca Stambler <rstambler@golang.org>
Trust: Peter Weinberger <pjw@google.com>

gopls/doc/settings.md

@@ -246,7 +246,7 @@ Default: `"Fuzzy"`.
 experimentalPostfixCompletions enables artifical method snippets
 such as "someSlice.sort!".
 
-Default: `false`.
+Default: `true`.
 
 #### Diagnostic
 

internal/lsp/source/api_json.go

@@ -265,7 +265,7 @@ var GeneratedAPIJSON = &APIJSON{
 					Keys:      nil,
 				},
 				EnumValues: nil,
-				Default:    "false",
+				Default:    "true",
 				Status:     "experimental",
 				Hierarchy:  "ui.completion",
 			},

internal/lsp/source/options.go

@@ -133,7 +133,7 @@ func DefaultOptions() *Options {
 					CompletionOptions: CompletionOptions{
 						Matcher:                        Fuzzy,
 						CompletionBudget:               100 * time.Millisecond,
-						ExperimentalPostfixCompletions: false,
+						ExperimentalPostfixCompletions: true,
 					},
 					Codelenses: map[string]bool{
 						string(command.Generate):          true,