Preserve leading whitespace in YAML literal blocks - #958

With INSTA_YAML_BLOCK_STYLE=1, a multiline string whose first line starts with whitespace is written as a literal block without an indentation indicator. YAML takes the indentation from that first line, so the leading whitespace is lost on read. If a later line is less indented, the file no longer parses: with description => " return x;\n }", the next run prints "Failed to parse snapshot file; a new snapshot will be generated" and the assertion fails even though the contents match.

This writes the indentation indicator when the first non-empty line starts with a space or tab, so description: |- becomes description: |2-. The vendored parser already reads the indicator, and output for all other strings is unchanged. With the variable set, assert_yaml_snapshot! uses the same emitter, so its output for such strings changes from |- to |2- and those snapshots need to be accepted once. Falling back to the quoted style for these strings would also work if you prefer that.

Ran:

• cargo test -p insta --lib --locked: 80 passed. Both new emitter tests fail on master.
• cargo test -p insta --all-features --locked: passes except the three test_globs_follow_links* tests, which fail the same way on master in this checkout.
• cargo clippy -p insta --all-targets --all-features -- --deny warnings and cargo fmt --all -- --check: clean.

Checked: a crate using with_settings!({description => ...}) with INSTA_YAML_BLOCK_STYLE=1 fails on master (the parse warning above, and an INSTA_REQUIRE_FULL_MATCH=1 mismatch for a uniformly indented description) and passes with this change.