feat: add Jump Back / Jump Forward to navigate the jump history

[RoiSoleil]

What

Adds Jump Back (Ctrl+Alt+Shift+←) and Jump Forward (Ctrl+Alt+Shift+→): navigate back and forward between the locations visited when jumping between a class under test and its test case (and vice versa), the way a web browser does.

Every jump (keyboard shortcut, editor / package explorer menus, code mining clicks) now remembers where the user jumped from:

• Jump Back returns to the previous location, restoring the exact cursor position
• Jump Forward goes forward again after having jumped back
• a new jump resets the forward history (browser behavior), consecutive duplicates are ignored, history is capped at 50 locations
• both actions are also available in the editor's MoreUnit contextual menu, and their shortcut is configurable in Preferences → Keys

Typical flow: Ctrl+Shift+J to the test method, write the test, Jump Back to continue coding the method under test at the exact place you left.

How

• new org.moreunit.navigation package: JumpLocation (compilation unit + offset) and JumpNavigationHistory (back/forward stacks, browser semantics)
• JumpActionExecutor registers every performed jump and executes the back/forward commands — for all jump entries: JumpCodeMining now delegates its jump to it instead of duplicating the code, so code mining jumps are followed too
• EditorUI.revealOffset(...) restores the cursor position when navigating
• EditorPartFacade.getTextSelection() is now null-safe (returns null instead of throwing when the editor has no text selection)

Testing

• 18 new unit tests: JumpNavigationHistoryTest (12) and JumpLocationTest (6)
• 4 new scenarios in JumpActionExecutorTest: jump registration (from editor and from package explorer entry points), back/forward round trip with cursor position restore, no-op when the history is empty
• 1 new SWTBot test (JumpNavigationTest) exercising the whole flow with the real shortcuts (jump → back → forward, asserting the editor and the cursor line), plus ShortcutStrategy.pressJumpBack/ForwardShortcut()
• full unit test run: 2043 tests, 0 failure (core.test 666, test 922, mock.test 433, mock.it 22)

⚠️ On my Linux/Wayland workstation, no SWTBot test can run (even unmodified BasicJumpTest fails there identically, with or without this branch), so JumpNavigationTest is expected to be exercised by the CI (windows-latest) only.

Documentation

help/documentation.html documents the new shortcuts next to the existing "Switching between test and class under test" section.