Standardise blank lines in the formatter

[ewels]

• Standardises the number of blank lines the formatter emits, resolving Standardise number of blank newlines language-server#115 and Blank line above process / workflow blocks language-server#150.
• The last of the formatter follow-ups deferred from Preserve comments in formatter #7346 in this stack.

The policy (PEP8/Ruff-style):

• exactly 2 blank lines around top-level block definitions (process, workflow, function, params, output, record, enum, agent), and between a definition and any adjacent declaration;
• simple declarations (includes, feature flags, legacy params) stay grouped, with runs of blank lines collapsed to at most one;
• process and workflow sections stay separated by exactly 1 blank line (this is Blank line above process / workflow blocks language-server#150);
• blank lines at the start of a block or section are removed;
• a shebang is followed by exactly 1 blank line;
• runs of blank lines anywhere else collapse to at most 1.
• Verbatim regions (fmt: off / fmt: skip, from the PR below this one) keep their original spacing untouched.

Notes:

• Stacked PR — targets formatter-fmt-directives (see Ability to skip formatting for specific sections of code language-server#75 ), which stacks on the wrapping and sort PRs below it. Review/merge those first; the diff here is only the blank-line change.

Two decisions worth a look in review, both intentional and easy to change:

• Exactly 2 blank lines around top-level definitions, and the feature is always on (not behind an option) — this is the most opinionated part and follows the PEP8 mapping requested in the issue.
• Config blocks are set off by 1 blank line rather than 2, since config files are denser than script definitions.

Mechanically, top-level spacing is now driven by policy in the declaration loop (blankLinesBetween(prev, decl)) rather than derived from each node's source position; the within-line collapse stays a stateless re-derivation from the source, and blank lines at a block/section start are dropped via a small atBlockStart flag set by each opener.

Example

Before:

include { FOO } from './modules/foo.nf'
process ALIGN {
    input:
    path reads


    script:
    'echo'
}



workflow {
    ALIGN(reads)
}

After:

include { FOO } from './modules/foo.nf'


process ALIGN {
    input:
    path reads

    script:
    'echo'
}


workflow {
    ALIGN(reads)
}

The include block is separated from the process by 2 blank lines (a definition), the run of blanks before script: collapses to the single blank that separates sections, and the 3 blank lines before workflow normalise to 2.

Assisted-by: Claude Opus 4.8 (Claude Code)