refactor: pluggable backend design

[AmaiKinono]

The goal of this PR is to

1. Separate the modules more clearly to ease the maintainence.
2. Make it easy to create a backend so other tools like GNU Global or cscope could benefit from the UI of Citre.

The codebase is divided into 4 parts:

1. Common Library: general utility functions. Including

   • citre-common-util.el: Helper functions of strings/files/processes...
   • citre-common-tag.el: The "tag" data structure and related functions.

2. UI library: functions that shows and allows the user to interact with a set of tags. Including

• citre-ui-jump.el
• citre-ui-peek.el

3. Frontend built on these 2 parts. Including

   • citre-backend-interface.el: Interface for defining a backend.
   • citre.el: User tools including commands, citre-mode, and capf/xref/imenu backend.

4. Backends: Including

   • citre-tags.el: The "tags" backend, which uses readtags to read from a tags file.
   • citre-global.el: The GNU Global backend.

The "tags" backend is built on:

• citre-readtags.el: The readtags program interface.
• citre-ctags.el: Finding & generating tags files.
• citre-lang-*.el: Language supports.
• citre-tags.el: The backend definition and language support framework.

The tests are rearranged, and the Makefile target "test-common" tests the common library, and "test-tags", "test-global" test the backends. When testing the backends, the script checks if their dependencies exist. If not, it shows a warning and skip the tests, so #104 is closed.

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TODOs:

• [x] It's good time to review the naming of all identifiers.
• [ ] citre-diagnostic which prints diagnostic information.
• [ ] Rethink citre-auto-enable-citre-mode (How do we support multiple backends that have different capabilities?)
• [ ] Improve the GNU Global backend.
• [ ] Update documents & commentaries.
• [ ] modify the CI in uctags repo to only test the tags backend after merge this.