logbuffer: split out LineBuffer

We want to be able to use similar line-oriented buffering in LogTree.
Rather than repeat ourselves, let's fact this out into a nice little
library.

Test Plan: Covered by existing logbuffer tests, added some extra linebuffer-specific ones.

X-Origin-Diff: phab/D636
GitOrigin-RevId: 38e832d323ed9f1723feaa9f9169caad18619e55
diff --git a/core/pkg/logbuffer/BUILD.bazel b/core/pkg/logbuffer/BUILD.bazel
index a53cb7a..fb7512a 100644
--- a/core/pkg/logbuffer/BUILD.bazel
+++ b/core/pkg/logbuffer/BUILD.bazel
@@ -2,14 +2,20 @@
 
 go_library(
     name = "go_default_library",
-    srcs = ["logbuffer.go"],
+    srcs = [
+        "linebuffer.go",
+        "logbuffer.go",
+    ],
     importpath = "git.monogon.dev/source/nexantic.git/core/pkg/logbuffer",
     visibility = ["//visibility:public"],
 )
 
 go_test(
     name = "go_default_test",
-    srcs = ["logbuffer_test.go"],
+    srcs = [
+        "linebuffer_test.go",
+        "logbuffer_test.go",
+    ],
     embed = [":go_default_library"],
     deps = ["@com_github_stretchr_testify//require:go_default_library"],
 )