Add branch/library management

[SanjulaGanepola]

Branch to library should use <branch-name>-<user>-<project>

Special variable

&BRANCHLIB will be a special variable that can be used in iproj.json or .ibmi.json. This variable will automatically be set based on the current branch.

Project Explorer new tree item

Simple case (no .ibmi.json files):

- Branches
  > Create branch (inline action on tree item)
  - feature/abcef (VSJFGJ49)
    > checkout to branch (inline action on tree item)
    > clear library (inline action on tree item)
    - MYPGM.PGM
    - OTHERTHING.FILE
  - bugfix/1234 (VSJT84HTO)
  - other/thing (VS48GH)
    - Branch library does not exist -> Create branch library (inline action on tree item)

Complex case (project contains .ibmi.json files):

- Branches
  - feature/abcd
    > checkout to branch (inline action on tree item)
    > clear library (inline action on tree item)
    - Objects (VSJFGJ49)
      - MYPGM.PGM -> mypgm.pgm.x
      - OTHERPGM.PGM -> Cannot find change in branch
    - Data (VSJFGJ4D)
      - OTHERFILE.FILE -> otherfile.dspf

New attribute in iproj.json

To hardcode/override library associated with branch:

{
  "branches": {
    "main": "MAINAPP",
    "dev": "DEVAPP",
    "feature/abcd": "VS208347"
  }
}

Useful references:
https://github.com/Microsoft/vscode/blob/main/extensions/git/README.md
https://github.com/codefori/vscode-ibmi/blob/master/src/api/local/git.ts