Testing fslexyacc caching + update devguide

[vzarytovskii]

Just testing the waters here, not entirely sure it's a good idea yet, maybe we should stick with some custom task to check the cache validity.

Parser and lexer outputs are cached in the repository and are regenerated automatically every time fslexyacc input files are changed (checked by fslexyacc itself).

However, they will not be regenerated if any dependent compiler libraries are changed. If you are making changes to the latter, and do not see your changes in the parser/lexer, you will need to manually regenerate the outputs. This can be done by removing src\.autogenerated\fslexyacc directory, and running build again.

Previously, it was enough to just clean your git folder (git clean -xdf of similar) or remove obj directory (or freshly clone the repo).

On my machine, this change saves roughly 20-25 seconds when building the compiler, if no changes were made to the lexer/parser.

Any thoughts about this? An alternative would be a custom task which hashes fslexyacc itself, its inputs and dependant compiler files and skips invoking fslexyacc alltogether if nothing changes, this will allow to save even more (by not invoking fslexyacc at all).

[github-actions[bot]]

:heavy_exclamation_mark: Release notes required

@vzarytovskii,

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