Support XDG directories for config and history files

[jgarber623]

Description

Addresses #1031 by implementing support for XDG directories in accordance with the rules discussed in the issue and summarized in my comment here.

This change should be non-breaking in that the code continues to prefer existing default configuration files at ~/.rdbgrc and ~/.rdbg_history.

If those files do not exist and XDG environment variables are configured, then configuration and history files in the relevant XDG directories are used.

• $XDG_CONFIG_HOME/rdbg/config (if file is present)
• $XDG_DATA_HOME/rdbg/history

Note that $XDG_CONFIG_HOME is typically $HOME/.config and that $XDG_DATA_HOME is typically $HOME/.local/share. These are customizable by users, but I believe those are the customary/default paths.

[launchable-app[bot]]

Launchable Report

:x: Test session #2769253 failed   details on CI
_{:bell: no issues :heavy_multiplication_x:118 tests failed :heavy_check_mark:179 tests passed}

:x: Test session #2769258 failed   details on CI
_{:bell: no issues :heavy_multiplication_x:121 tests failed :heavy_check_mark:176 tests passed}

:x: Test session #2769260 failed   details on CI
_{:bell: no issues :heavy_multiplication_x:118 tests failed :heavy_check_mark:179 tests passed}

Passed test sessions

:white_check_mark: Test session #2769168 passed   details on CI
:white_check_mark: Test session #2769169 passed   details on CI
:white_check_mark: Test session #2769171 passed   details on CI
:white_check_mark: Test session #2769172 passed   details on CI
:white_check_mark: Test session #2769176 passed   details on CI
:white_check_mark: Test session #2769180 passed   details on CI
:white_check_mark: Test session #2769182 passed   details on CI
:white_check_mark: Test session #2769183 passed   details on CI
:white_check_mark: Test session #2769184 passed   details on CI
:white_check_mark: Test session #2769185 passed   details on CI
:white_check_mark: Test session #2769186 passed   details on CI
:white_check_mark: Test session #2769189 passed   details on CI
:white_check_mark: Test session #2769201 passed   details on CI
:white_check_mark: Test session #2769215 passed   details on CI
:white_check_mark: Test session #2769220 passed   details on CI
:white_check_mark: Test session #2769222 passed   details on CI
:white_check_mark: Test session #2769223 passed   details on CI
:white_check_mark: Test session #2769224 passed   details on CI
:white_check_mark: Test session #2769240 passed   details on CI
:white_check_mark: Test session #2769257 passed   details on CI

Build: refs_pull_1055_merge_55f66240abf152a91a42df8aa4d05e2a25387bdd

[jgarber623]

Hello! I don't have the ability to re-run the failed checks and it's not immediately clear to me why those failed (and why Ruby 3.2 and 3.3 passed).

For someone more knowledgeable, do those failures look to be caused by changes in this pull request? Should I update this branch with a merge commit and have someone re-run the test suite?