feat(lua): vim.text.indent()

Problem:
Indenting text is a common task in plugins/scripts for
presentation/formatting, yet vim has no way of doing it (especially
"dedent", and especially non-buffer text).

Solution:
Introduce `vim.text.indent()`. It sets the *exact* indentation because
that's a more difficult (and thus more useful) task than merely
"increasing the current indent" (which is somewhat easy with a `gsub()`
one-liner).

src/nvim/CMakeLists.txt

@@ -342,6 +342,7 @@ set(LUA_KEYMAP_MODULE_SOURCE ${NVIM_RUNTIME_DIR}/lua/vim/keymap.lua)
 set(LUA_LOADER_MODULE_SOURCE ${NVIM_RUNTIME_DIR}/lua/vim/loader.lua)
 set(LUA_OPTIONS_MODULE_SOURCE ${NVIM_RUNTIME_DIR}/lua/vim/_options.lua)
 set(LUA_SHARED_MODULE_SOURCE ${NVIM_RUNTIME_DIR}/lua/vim/shared.lua)
+set(LUA_TEXT_MODULE_SOURCE ${NVIM_RUNTIME_DIR}/lua/vim/text.lua)
 
 file(GLOB API_HEADERS CONFIGURE_DEPENDS api/*.h)
 list(REMOVE_ITEM API_HEADERS ${CMAKE_CURRENT_LIST_DIR}/api/ui_events.in.h)
@@ -624,6 +625,7 @@ add_custom_command(
       ${LUA_DEFAULTS_MODULE_SOURCE} "vim._defaults"
       ${LUA_OPTIONS_MODULE_SOURCE} "vim._options"
       ${LUA_SHARED_MODULE_SOURCE} "vim.shared"
+      ${LUA_TEXT_MODULE_SOURCE} "vim.text"
   DEPENDS
     ${CHAR_BLOB_GENERATOR}
     ${LUA_INIT_PACKAGES_MODULE_SOURCE}
@@ -637,6 +639,7 @@ add_custom_command(
     ${LUA_DEFAULTS_MODULE_SOURCE}
     ${LUA_OPTIONS_MODULE_SOURCE}
     ${LUA_SHARED_MODULE_SOURCE}
+    ${LUA_TEXT_MODULE_SOURCE}
   VERBATIM
 )