Expose `org.jenkins.ui.symbol.Symbol` (#6659)

* Expose org.jenkins.ui.symbol.Symbol

* Exposes the existing IconSet#getSymbol for plugins by adding a builder
pattern and moving it to its own package and class to separate the
symbol framework from the Icon one.
* Added coverage to the existing method including symbol lookup from
  plugins

* Fixes JENKINS-68805 and pick up #6685

* Clean up

* Rename method since the intent slightly changed

* Simplify

* Make this compile

* Remove support for legacy names

* Fix reviews

* Add assertions for the symbol content used for testing

* Read science svg from filesystem

* Add withRaw to parse a raw selector

Co-authored-by: Daniel Beck <daniel-beck@users.noreply.github.com>
Co-authored-by: Basil Crow <me@basilcrow.com>