[palette-] prevent interpretation of markup in go-col-name

[midichef]

The palette choosers for & + and gc have some minor display inconsistencies with the way longnames are displayed in exec-longname. The first two commits in this PR fix those inconsistencies.

The third commit fixes a bug in gc where a column name had markup interpreted, so 'Customer[:5]' was shown as Customer, and cell contents had markup interpreted, so a cell of [:bold]asdf[/] was drawn as asdf in bold. However, to properly display the column name as a literal with no markup, I had to turn off the bolding of the column name that was just added in the first commit (40d4243).

[midichef]

Hmm. Turning off formatting in go-col-name was a bad idea. Now it fails to interpret the coloring of the match. So for a colname like Customer. When I type C I see: [:match]C[/]ustomer. So we have to trade off between: coloring matches, vs. correctness for column names that contain pseudomarkup like [:text].

What would be a good way to escape visidata markup?

[midichef]

Perhaps we can escape vd markup by wrapping every open bracket [ between some no-op markup that has no color bound to it, like: [:literal] and [:]. I think it should work, judging by the working of iterchunks() and internal_markup_re. But the idea needs to be double-checked for validity, in case I have missed any important case.

def escape_vdcode(s:str) -> str:
    return re.sub(r'\[', '[:literal][[:]', s)

def unescape_vdcode(s:str) -> str:
    return re.sub(r'\[:literal\]\[\[:\]', '[', s)

If this works, I would discard the changes in f19d7ba and change go-col-name to escape the colname and the cell contents.