fix(sql): allow JOINs without an explicit alias on the joined table

[fupelaqu]

Closes #95.

Join.validate() rejected any join whose joined table had no alias — but standard SQL doesn't require one. The bare table name is a valid qualifier (PostgreSQL, MySQL, DuckDB, ClickHouse, SQL Server). Removing the rule surfaces two latent issues that have to be fixed together:

1. From.scala — Join.validate() Drop the two unconditional alias-required branches (the explicit alias match + the redundant case j if alias.isEmpty arm in the second match). Keep the ON-clause guard for non-CROSS joins. The downstream sites that key off the join alias already fall back to the source name when no alias is set (From.tableAliases, From.joinAliases, From.unnestAliases, JoinKey.apply, Unnest.innerHitsName), so no call site needs to change.

2. From.scala — StandardJoin.update() Stop calling source.update(request). The source of a JOIN is a TABLE name, not a column expression. With the alias requirement gone, From.tableAliases now contains a customers → customers self-mapping for the alias-less join — which made GenericIdentifier.update treat name="customers" as alias.column, take parts.tail.mkString(".") of a single-part name, and end up with name="". The ON clause still gets .update(request) so column resolution within ON works.

3. Parser.scala — reservedKeywords Add inner, left, right, full, cross, outer, on. The alias regex's negative-lookahead filter excluded from/join/etc. but not the join-type keywords — so without an alias on orders, the regex greedily consumed LEFT as the alias, leaving the join rule to parse a junk JOIN with no joinType. Functions like LEFT(x, 5) and RIGHT(x, 3) are parsed by their dedicated function parsers (not via identifierRegex/regexAlias), so they continue to work — verified by ParserSpec line 225's SELECT that exercises both.

ParserSpec gets three regression cases:

• parses LEFT JOIN customers ON orders.customer_id = customers.id and exposes join.alias = None and join.source.name = "customers".
• alias-less joined table is reachable in From.tableAliases by its bare name ("customers" -> "customers").
• a JOIN with no ON clause still fails — only the alias rule was relaxed, the ON rule was not.