[Bug] Executing `TableFormat` inserts an empty whitespace column at the beginning of the table each time it is called

[whisperity]

Given

~ % nvim --version
NVIM v0.10.4
Build type: Release
LuaJIT 2.1.1736781742

tabular % git -P log -1
commit 12437cd1b53488e24936ec4b091c9324cafee311 (grafted, HEAD -> master, origin/master, origin/HEAD)
Author: Matt Wozniski <[email protected]>
Date:   Wed Jul 3 12:03:37 2024 -0400

    Fix duplicate :Tabularize tag in docs

vim-markdown % git -P log -1
commit 8f6cb3a6ca4e3b6bcda0730145a0b700f3481b51 (grafted, HEAD -> master, origin/master, origin/HEAD)
Author: Hiroshi Shirosaki <[email protected]>
Date:   Fri Sep 20 15:23:57 2024 +0900

    Merge pull request #636 from KSR-Yasuda/bugfix/CodeFenceHandling

    Code fence handling breaks if `~~~` appears in the fence

The following simple source file:

| A | B | C |
|---|---|---|
| a | b | c |

if I position into the table and execute :TableFormat<CR>, the table is transformed as such:

 | A | B | C |
 |---|---|---|
 | a | b | c |

subsequent executions of TableFormat results in additional prefix whitespace. 4 executions in total adds 4 whitespace, which is problematic because 4 prefixing whitespace is the indentation for a code block:

    | A | B | C |
    |---|---|---|
    | a | b | c |

and will be rendered broken:

| A | B | C |
|---|---|---|
| a | b | c |

---

If alignment characters are used in the header row separator, the situation is much worse:

| A | B | C |
|:--|:-:|--:|
| a | b | c |

after 1 application:

 | A  | B   | C  |
 |:---|:---:|---:|
 | a  | b   | c  |

after 2 applications:

  | A  | B   | C  |
  |:---|:---:|---:|
  | a  | b   | c  |

(subsequent applications will only increase the prefix whitespace and no longer change the table itself).

[alerque]

I'm not able to look into this right now, but skimming your report does suggest this is a legitimate issue. I'd be happy to try to facilitate a PR getting merged if you or someone else can contribute a fix.