Add export() to CMakeLists.txt enabling importing targets

[martinmoene]

Inspired by expected-lite PR 39.

Add the following code to the endof ./CMakeLists.txt:

export(
    EXPORT       ${package_target}
    NAMESPACE    ${package_nspace}::
    FILE         "${CMAKE_CURRENT_BINARY_DIR}/${package_name}-targets.cmake"
)

Plan:

• any-lite
• atomic-lite (being developed)
• bit-lite
• boolean-lite
• byte-lite
• expected-lite
• invoke-lite (NA)
• observer-ptr-lite
• optional-bare
• optional-fun-lite
• optional-lite
• ring-span-lite
• scope-lite
• span-lite
• status-value-lite
• string-view-lite
• type-lite
• value-ptr-lite
• variant-lite