guide: initial type sheet

visidata/guides/TypesSheet.md

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+# Types
+
+Columns usually begin as untyped (`anytype`), but can be set.
+
+- {help.commands.type-any}
+- {help.commands.type-currency}
+- {help.commands.type-date}
+- {help.commands.type-float}
+- {help.commands.type-int}
+- {help.commands.type-len}
+- {help.commands.type-string}
+
+*Note*: `float` uses the decimal separator; `currency` removes non-numeric characters and parses as float; `date` parses dates into ISO8601 format; `len` formats the cell value to the length of the content.