feat: point config errors to the line of the invalid entry

[iamralch]

A half-deleted secret, e.g. a - kind: env line left without name and path, reported only secret at profile 'p' index 3 missing 'name' field: a 0-based index, no line number, and only the first missing field.

Config errors for profiles and secrets with missing fields now report the line, the secret's name (or its 1-based position), the fields that are there, and every missing field:

secret #4 in profile 'personal' (line 17, kind: env) is missing 'name' and 'path'
secret 'A' in profile 'p' (line 8, kind: env) is missing 'path'
profile #1 (line 3) is missing 'name' and 'provider'
profile 'p' (line 3) is missing 'provider'

• serde_yaml_ng keeps no positions, but it stamps errors with the position of the mapping they are raised in. So when validation fails, line_of reads the YAML again up to the entry and raises an error there on purpose to get its line. Valid configs are parsed once, as before, and no dependency is added.
• Missing fields are now checked before an invalid kind or variable name, so an entry with both problems reports the missing fields first.
• A profile with no name but a leftover zsh-op-style account: key now gets the missing name/provider message. Named profiles still get the hint to move account into the provider.

Tested with unit tests for the messages, a half-deleted entry among valid ones, an empty entry, and line_of on block- and flow-style YAML (all 166 tests pass, clippy clean). Also tried the binary on a broken config.

Closes #35