tt CLI (2.7): allow multiple directories with modules in `tt.yaml`

Product: tt cli
Since: tt 2.7.0
Audience/target: all
Root document: https://www.tarantool.io/ru/doc/latest/reference/tooling/tt_cli/
SME: @ mandesero @ oleg-jukovec @ dmyger

Details

Duplicate issue: https://github.com/tarantool/doc/issues/4664

The logic of working with configuration file tt.yaml has been updated,
now modules/directory can be specified both as a single line and
as a list.

Usage example
Old behavior:

modules:
  directory: modules

Possibility to specify a list of directories:

modules:
  directory:
  - modules
  - /ext/path/modules
  - other_modules

If a relative path is specified, the search is performed in
the subfolders below, relative to the tt.yaml file. If an absolute
path is specified, external modules are searched according to
the specified path.
Requested by @dmyger in https://github.com/tarantool/tt/commit/96db0688623bf0a8072b3c985eb20c3d1b5df617. and https://github.com/tarantool/tt/commit/139119eedc281b5b8545b7d8e0f492f55d965b7e

Start with the tt CLI documentation at the linked root document and check duplicate issue 4664 for the current scope. Update the tt.yaml modules.directory section to document both a single directory and a list, including relative paths below tt.yaml and absolute external paths; the usage examples should match these behaviors.