Add a section to specify plotline/marklines/gridlines (same concept for different libraries)

Problem/Motivation

A common need in charting is the ability to visualize the relationship between the series values and a given value, such as a target, upper or lower limit, or fixed comparison. These can sometimes be plotted as series, but, especially when they are fixed, they are generally better as a plotline (this is the Highcharts term for this concept).

A quick search reveals that there is wide support for this concept among the different libraries supported by the Charts module:

Proposed resolution

Add a conditional "Plot lines" section to the chart settings form (views, field, and block), shown only for the packaged libraries that support the concept (Highcharts, Chart.js, C3, Billboard). Contrib libraries are out of scope for this issue.

The section follows the same table format as the views data provider table. Each row is a candidate source with an enable checkbox, an orientation select (horizontal = value/y-axis, vertical = category/x-axis), a color, and a label:

• In views, the sources are the view fields; the line is drawn at the value the field provides.
• In field, block, and canvas block, the sources are the data series; an enabled series renders as a plot line instead of a regular series.

Charts does not aggregate the value. It is expected to arrive ready-made from the data source — Views aggregation, a module such as Views Descriptive Statistics, or a precomputed column (e.g. a threshold column in a CSV) — so the first value of the field/series is used.

The value is mapped to each library's native equivalent: Highcharts plotLines, C3/Billboard grid.x/y.lines, and Chart.js annotation lines. Because plot lines do not extend an axis's range, Highcharts and Chart.js also receive soft/suggested bounds so a line outside the data range stays visible.

Remaining tasks

• Review the MR.
• Manual testing across the four supported libraries in views, field, and block contexts.
• Decide whether to give C3/Billboard the same in-range guarantee (they have no soft-bounds equivalent; would require setting an explicit axis min/max).
• Decide whether plot line selection should key off series name rather than positional index, to survive column reordering on CSV import.

User interface changes

New collapsible "Plot lines" section in the chart settings form, rendered only for libraries that support plot lines. The section updates over AJAX when rows/columns are added to the data collector table, so it no longer requires a save-and-reopen.

API changes

• New #plot_lines property on the chart_xaxis / chart_yaxis render elements (array of items with value, label, color).
• New ChartBase::supportsPlotLines() method (default FALSE; overridden to TRUE by the four packaged libraries). Not added to ChartInterface, to avoid breaking plugins that implement the interface directly; callers guard with method_exists().
• New #ajax_companions property on the data collector table element, letting sibling elements derived from the table refresh in the same AJAX response.
• New Util::firstNumericValue() helper.

Data model changes

New plot_lines.sources mapping added to the chart settings config schema (and the canvas block schema), keyed by field/series, each storing enabled, orientation, color, and label_text. Additive only; no update hook required.