Add a 'theme' for OMP, so the internal themes don't clash with it. (#2100)

* Add a 'theme' for OMP, so the internal themes don't clash with it.

* Add theme to clean_files

* Add screenshot to the docs

* Correct the name of the default theme in the docs.

* keeping it cleaner

Co-authored-by: Ira Abramov <github@ira.abramov.org>

themes/oh-my-posh/oh-my-posh.theme.bash

@@ -0,0 +1,8 @@
+# shellcheck shell=bash
+
+if _command_exists oh-my-posh; then
+	export POSH_THEME=${POSH_THEME:-https://raw.githubusercontent.com/JanDeDobbeleer/oh-my-posh/v$(oh-my-posh --version)/themes/jandedobbeleer.omp.json}
+	eval "$(oh-my-posh --init --shell bash --config "${POSH_THEME}")"
+else
+	_log_warning "The oh-my-posh binary was not found on your PATH. Falling back to your existing PS1, please see the docs for more info."
+fi