This PR adjusts the logic for skipping formatting so that a fmt: skip can affect multiple statements if they lie on the same line.

Specifically, a fmt: skip comment will now suppress all the statements in the suite in which it appears whose range intersects the line containing the skip directive. For example:

x=[
'1'
];x=2 # fmt: skip

remains unchanged after formatting.

(Note that compound statements are somewhat special and were handled in a previous PR - see #20633).

Closes #17331 and #11430.

Simplest to review commit by commit - the key diffs of interest are the commit introducing the core logic, and the diff between the snapshots introduced in the last commit (compared to the second commit).

Implementation

On main we format a suite of statements by iterating through them. If we meet a statement with a leading or trailing (own-line)fmt: off comment, then we suppress formatting until we meet a fmt: on comment. Otherwise we format the statement using its own formatting rule.

How are fmt: skip comments handled then? They are handled internally to the formatting of each statement. Specifically, calling .fmt on a statement node will first check to see if there is a trailing, end-of-line fmt: skip (or fmt: off/yapf: off), and if so then write the node with suppressed formatting.

In this PR we move the responsibility for handling fmt: skip into the formatting logic of the suite itself. This is done as follows:

• Before beginning to format the suite, we do a pass through the statements and collect the data of ranges with skipped formatting. More specifically, we create a map with key given by the first skipped statement in a block and value a pair consisting of the last skipped statement and the range to write verbatim.
• We iterate as before, but if we meet a statement that is a key in the map constructed above, we pause to write the associated range verbatim. We then advance the iterator to the last statement in the block and proceed as before.

Addendum on range formatting

We also had to make some changes to range formatting in order to support this new behavior. For example, we want to make sure that

<RANGE_START>x=1<RANGE_END>;x=2 # fmt: skip

formats verbatim, rather than becoming

x = 1;x=2 # fmt: skip

Recall that range formatting proceeds in two steps:

1. Find the smallest enclosing node containing the range AND that has enough info to format the range (so it may be larger than you think, e.g. a docstring has enclosing node given by the suite, not the string itself.)
2. Carve out the formatted range from the result of formatting that enclosing node.

We had to modify (1), since the suite knows how to format skipped nodes, but nodes may not "know" they are skipped. To do this we altered the visit_body bethod of the FindEnclosingNode visitor: now we iterate through the statements and check for skipped ranges intersecting the format range. If we find them, we return without descending. The result is to consider the statement containing the suite as the enclosing node in this case.