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.
Convert the Event and Format enums from fixed-size Zig enums to
lib.Enum so they are C ABI compatible when targeting C. The motion
method on Event becomes a free function eventIsMotion since lib.Enum
types cannot have declarations.
Move MouseEvent and MouseFormat out of Terminal.zig and MouseShape out
of mouse_shape.zig into a new mouse.zig file. The types are named
without the Mouse prefix inside the module (Event, Format, Shape) and
re-exported with the prefix from terminal/main.zig for external use.
Update all call sites (mouse_encode.zig, surface_mouse.zig, stream.zig)
to import through terminal/main.zig or directly from mouse.zig. Remove
the now-unused mouse_shape.zig.
