Each C API file independently imported ../../lib/allocator.zig as
lib_alloc. Now that terminal/lib.zig re-exports the allocator module
as lib.alloc, use that instead. This removes the redundant import
and keeps all lib dependencies flowing through the single lib.zig
entry point.
Previously every file in the terminal package independently imported
build_options and ../lib/main.zig, then computed the same
lib_target constant. This was repetitive and meant each file needed
both imports just to get the target.
Introduce src/terminal/lib.zig which computes the target once and
re-exports the commonly used lib types (Enum, TaggedUnion, Struct,
String, checkGhosttyHEnum, structSizedFieldFits). All terminal
package files now import lib.zig and use lib.target instead of the
local lib_target constant, removing the per-file boilerplate.
