Refactor code for parsing ascii-style tables

[rw251]

There are 4 places in the code where ascii-style tables e.g.

 x |  y
---+-----
 1 | 101
 2 | 201

are parsed into python objects. This should be refactored so the parsing is common to all uses.

Current locations of ascii table parsing are (roughly):

ehrql/ehrql/docs/specs.py

Lines 210 to 211 in 42b273b

	def parse_table(s):
	"""Return data in table as list of lists."""

ehrql/ehrql/query_engines/in_memory_database.py

Lines 64 to 65 in 42b273b

	def parse(cls, s):
	"""Create instance by parsing string.

ehrql/ehrql/query_engines/in_memory_database.py

Lines 147 to 148 in 42b273b

	def parse(cls, s):
	"""Create instance by parsing string.

ehrql/tests/spec/conftest.py

Lines 94 to 95 in 42b273b

	def parse_row(column_types, col_names, line):
	"""Parse string containing row data, returning list of values.