ext: Rename go_strings_library target to go_default_library

[martinxsliu]

Hi team, this PR renames the //ext:go_strings_library Bazel target to //ext:go_default_library. If we are concerned about backwards compatibility, we alternatively add an alias rule instead.

The go_default_library target name is the default name generated by Gazelle and is also the name used by all other go_library targets in this package. Other than being conventional, this custom named target makes using the ext package transitively by other packages (example) difficult. This is because Gazelle generates deps that assume the go_default_library target name, so to work around this we would need to supply a patch file for this repository that adds an alias in.

[TristonianJones]

/gcbrun

[TristonianJones]

Thanks!