Fix issue #432: restore natural table width and horizontal scrolling

[samqbush]

Problem

Wide Markdown tables are squeezed to the visible pane width with no horizontal scrolling. Old MacDown v0.7.2 rendered wide tables at natural width and allowed horizontal scrolling via WebKit's NSScrollView.

Closes #432

Root Causes (both verified in main)

Cause 1 — GitHub-2020.css default theme body table rule

width: max-content was immediately cancelled by max-width: 100%, clamping the table to the pane width so overflow: auto had nothing to scroll.

Cause 2 — export.css (loads last, all themes) td, th rule

word-break: break-word forced mid-word cell breaks universally, collapsing column widths across every theme regardless of Cause 1.

Changes

MacDown/Resources/Styles/GitHub-2020.css

/* Before */
body table {
  display: block;
  width: 100%;
  width: max-content;
  max-width: 100%;      /* ← kills scrolling */
  overflow: auto;
  font-variant: tabular-nums;
}

/* After */
body table {
  display: block;
  width: max-content;
  min-width: 100%;      /* fill pane, allow growth */
  overflow-x: auto;
  font-variant: tabular-nums;
}

MacDown/Resources/Extensions/export.css

/* Before */
td, th {
    word-break: break-word;     /* ← collapses columns */
    overflow-wrap: break-word;
}

/* After */
td, th {
    overflow-wrap: break-word;  /* long URLs still wrap; cells size naturally */
}

MacDownTests/Fixtures/tables.md + tables.html

Added a 10-column wide-table regression fixture to guard against re-introducing either regression.

Testing

• All 56 tests in MPMarkdownRenderingTests and MPHTMLExportTests pass (including the new wide-table fixture).